Cascade redox flow battery systems

ABSTRACT

A reduction/oxidation (“redox”) flow battery system includes a series of electrochemical cells arranged in a cascade, whereby liquid electrolyte reacts in a first electrochemical cell (or group of cells) before being directed into a second cell (or group of cells) where it reacts before being directed to subsequent cells. The cascade includes 2 to n stages, each stage having one or more electrochemical cells. During a charge reaction, electrolyte entering a first stage will have a lower state-of-charge than electrolyte entering the nth stage. In some embodiments, cell components and/or characteristics may be configured based on a state-of-charge of electrolytes expected at each cascade stage. Such engineered cascades provide redox flow battery systems with higher energy efficiency over a broader range of current density than prior art arrangements.

RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 12/883,511 filed Sep. 16, 2010, which is a Divisional of U.S.patent application Ser. No. 12/498,103, filed on Jul. 6, 2009, now U.S.Pat. No. 7,820,321, which claimed the benefit of priority to U.S.Provisional Patent Application No. 61/078,691 filed Jul. 7, 2008 andU.S. Provisional Application No. 61/093,017 filed Aug. 29, 2008, theentire contents of all four of which are hereby incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Inventions conceived after the filing of the priority application (U.S.patent application Ser. No. 12/498,103, filed on Jul. 6, 2009) that areincluded in this continuation-in-part patent application were made withGovernment support under DE-OE0000225 “Recovery Act—Flow BatterySolution For Smart Grid Renewable Energy Applications” awarded by the USDepartment of Energy (DOE). The Government has certain rights in suchinventions. However, the Government does not have rights in U.S. Pat.No. 7,820,321 which was conceived and filed without Government support,nor in the direct continuation and divisional applications thereof.

FIELD OF INVENTION

This invention generally relates to energy storage technologies, andmore particularly to redox flow battery energy storage systems andapplications.

BACKGROUND

Many renewable energy technologies, while economically viable andenvironmentally beneficial, suffer from the disadvantage of periodic andunpredictable power generation. To enable such renewable energytechnologies to expand, large scale energy storage systems are required.Additionally, many conventional electric generation technologies, suchas coal and nuclear power plants, as well as promising alternativeenergy generation technologies, such as fuel cells, function best whenoperated at constant power, and thus can benefit from energy storagesystems that can deliver peak power when needed and store energy duringoff-peak hours.

SUMMARY

Reduction/oxidation (redox) flow batteries of the various embodimentsoffer one large-capacity energy storage solution. Redox flow batteriesare electrochemical energy storage systems which store electrical energyin chemical reactants dissolved in liquids. Thus, the energy storagecapacity of a flow battery is theoretically limited only by tank size.An efficient and cost-effective redox flow battery system of the variousembodiments would provide a needed addition to electrical grid systems.

A redox flow battery storage system of the various embodiments storeelectrical energy in electrolyte species. The redox flow battery storagesystem includes at least one redox flow battery stack assembly includinga number of layers with each layer including multiple independent cellsin a cascade orientation along the reactant flow path. The cells of theredox flow battery stack assembly may configured to increase electricalstorage efficiency with the state-of-charge of reactants expected ineach cell. Reactants may be heated to increase battery efficiency, withwaste heat from energy sources or uses of energy from the battery orsystem providing the thermal energy to heat the reactants. By adjustingthe size of tanks for storing the reactants and adding redox flowbattery stack assemblies, the storage system can be sized to supportmulti-megawatt implementations suitable for use with power gridapplications. Thermal integration with energy generating systems, suchas fuel cell, wind and solar systems, further maximizes total energyefficiency. The redox flow battery system of the various embodiments canalso be scaled down to smaller applications, such as a gravity feedsystem suitable for small and remote site applications.

Redox flow battery stack assemblies of the various embodiments comprisea plurality of electrochemical cells arranged in hydraulic series toform a cascade arrangement. In some embodiments, such cascadearrangements comprise cells configured for a portion of astate-of-charge gradient that the cell is expected to experience.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate exemplary embodiments of theinvention, and, together with the general description given above andthe detailed description given below, serve to explain features of theinvention.

FIG. 1 is a system diagram of an embodiment large stack redox batterysystem showing a cross sectional schematic illustration of a redoxbattery stack from a first viewing perspective.

FIG. 2 is cross sectional schematic illustration of an embodiment redoxbattery stack cell layer of three cells from a second viewingperspective.

FIG. 3A is a cross section diagram of an embodiment single redox batterycell from a third viewing perspective.

FIG. 3B is an exploded view of an embodiment single redox battery cell.

FIG. 4 illustrates two chemical equations of a chemical reaction thatmay be employed within a redox battery embodiment.

FIG. 5 is a graph of design parameters that may be implemented within aredox battery system embodiment.

FIG. 6 is a graph of electrical potential versus current of a redoxbattery.

FIG. 7A is a schematic diagram of a redox flow battery stack accordingto an embodiment.

FIG. 7B is an assembly drawing illustrating how cell layers can beassembled into a flow battery stack according to an embodiment.

FIG. 7C is assembly drawing illustrating how cell layers can beassembled into a flow battery stack according to an alternativeembodiment.

FIG. 8 is an illustration of a separator portion of a redox battery cellaccording to an embodiment.

FIG. 9 is system diagram of a wind farm system implementation embodimentwith thermal integration.

FIG. 10 is system diagram of a solar power system implementationembodiment with the electrolyte fluid heated directly by the solarpanels.

FIG. 11 is system diagram of an alternative solar power systemembodiment with thermal integration via a secondary fluid flowing aroundthe power stack.

FIG. 12 is a table of system design parameters according to anembodiment.

FIG. 13A is a system block diagram of an embodiment system including aredox flow battery used as an AC to DC power conversion/isolation directcurrent electrical power source.

FIG. 13B is a system block diagram of an embodiment system including aredox flow battery used as a surge electrical power source forrecharging electric vehicles.

FIG. 13C is a system block diagram of an alternative embodiment systemincluding a redox flow battery used as a surge electrical power sourcefor recharging electric vehicles.

FIG. 13D is a system block diagram of an embodiment system including aredox flow battery used as a electrical power storage and load followingpower management system enabling a fuel cell to provide AC power to anelectrical grid.

FIG. 14 is a cross sectional component block diagram of a gravity drivenredox flow battery embodiment.

FIGS. 15A-15C are a series of cross sectional component block diagramsof a gravity driven redox flow battery embodiment illustrating atransition from charging mode to discharging mode.

FIGS. 16A-16C are micrographs showing representative separator materialssuitable for use in each of three cells of a three-cell stack cell layerredox flow battery embodiment.

FIG. 17 is a system diagram of an embodiment large stack redox batterysystem showing a cross sectional schematic illustration of a redoxbattery stack with reactant storage tanks including tank separators.

FIG. 18 is a graph of battery cell potential versus time illustratingeffects of mixing of charged and discharged reactants.

FIGS. 19A-19F are cross sectional diagrams of an embodiment electrolytestorage tank including a tank separator illustrating movement of thetank separator through a charging or discharging cycle.

FIGS. 20A-20F are cross sectional diagrams of an embodiment electrolytestorage tank including a tank separator illustrating movement of thetank separator through a charging or discharging operations.

FIG. 21 is a schematic cross-sectional view of an electrochemical cellin a redox flow battery.

FIG. 22 is an exploded perspective view of one embodiment of a block ofelectrochemical cells configured for use in a redox flow battery system.

FIG. 23 is a perspective view of an assembled cell block similar to thatof FIG. 3.

FIG. 24 is a schematic illustration of an embodiment of a flow batterystack assembly comprising a four-stage cascade arrangement.

FIG. 25 is a perspective view of a flow battery stack assemblycomprising cell blocks in a six-stage cascade arrangement.

FIG. 26 illustrates a pair of design trend lines and parameters for usein optimizing cells in a flow battery stack assembly with a cascadearrangement.

FIGS. 27A-27D illustrate graphs of optimum state-of-charge vs. cascadestage position for embodiments of engineered cascade flow battery stackassemblies.

FIG. 28 illustrates an embodiment of a direct cascade flow battery stackassembly with direct hydraulic connections.

FIG. 29 is a schematic illustration of one embodiment of hydraulicconnections for a single electrolyte in a three-stage cascade flowbattery stack assembly.

FIG. 30 is a schematic illustration of an alternative embodiment ofhydraulic connections for a single electrolyte in a three-stage cascadeflow battery stack assembly.

FIG. 31 is a graph of energy efficiency and current density for aplurality of redox flow battery stack assemblies.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.References made to particular examples and implementations are forillustrative purposes, and are not intended to limit the scope of theinvention or the claims.

As used herein, the terms “about” or “approximately” for any numericalvalues or ranges indicates a suitable temperature or dimensionaltolerance that allows the part or collection of components to functionfor its intended purpose as described herein.

The term “engineered cascade flow battery” is used herein to refergenerally to a cascade flow battery in which cells, stages and/or arrayswithin the battery are configured in terms of materials, design shapesand sizes, reactant flow, and/or other design variables based on anexpected condition of reactants (e.g., the state of charge ofelectrolytes) so as to increase the battery's performance (e.g., energystorage efficiency, power generation efficiency, reduced electrolytebreakdown, reduced hydrogen generation, or other performance) over thatachievable in a cascade flow battery in which all cells, stages and/orarrays along the reactant flow path are substantially the same as oneanother. References to “optimized” or “optimum” are merely intended toindicate design parameters which may be controlled or varied in anengineered cascade flow battery in order to improve performance and todistinguish the embodiments from designs in which there is noconfiguration based on expected local properties of reactants. Use ofthese terms is not intended to imply or require that the any cells,stages and/or arrays or components thereof are designed for the bestpossible or theoretical performance.

As used herein the phrase “state of charge” and its abbreviation “SOC”refer to the chemical species composition of at least one liquidelectrolyte. In particular, state of charge and SOC refer to theproportion of reactants in the electrolyte that have been converted(e.g. oxidized or reduced) to a “charged” state from a “discharged”state. For example, in a redox flow battery based on an Fe/Cr redoxcouple, the state of charge of the catholyte (positive electrolyte) maybe defined as the percent of total Fe which has been oxidized from theFe²⁺ state to the Fe³⁺ state, and the state of charge of the anolyte(negative electrolyte) may be defined as the percent of total Cr whichhas been reduced from the Cr³⁺ state to the Cr²⁺ state. In someembodiments, the state of charge of the two electrolytes may be changedor measured independent of one another. Thus, the terms “state ofcharge” and “SOC” may refer to the chemical composition of only one orof both electrolytes in an all-liquid redox flow battery system. Theskilled artisan will also recognize that the state of charge of one orboth electrolytes can be changed by processes other than electrochemicalprocesses (e.g., by adding quantities of one or more reactant species).

The embodiments provide an energy storage system based upon areduction/oxidation (redox) flow battery system that is suitable forstoring and delivering electric energy under a wide variety ofconditions. Electric energy stored by the redox flow battery system canbe produced from a wide variety of electric generation or conversionmethods, including hydroelectric, natural gas, coal, gasoline, diesel orother liquid petroleum fuel, nuclear, wave power, tidal power, solar,thermal energy, wind, etc. The redox flow battery systems of the variousembodiments are also capable of delivering stored energy to a widevariety of loads, including a distributed electrical grid, a datacenter, an irrigation pump, a cellular telephone station, another energystorage system, a vehicle, a vehicle charging system, a building, or anyother electrical load.

Flow batteries are electrochemical energy storage systems in whichelectrochemical reactants are dissolved in liquid electrolytes(sometimes referred to herein collectively as “reactant” or“reactants”), which are pumped through reaction cells (referred toherein as “cells”) where energy is either added to or extracted from thebattery. In applications where megawatts of electrical energy must bestored and discharged, a redox flow battery system can be expanded tothe required energy storage capacity by increasing tank sizes andexpanded to produce the required output power by adding electrochemicalcells or cell blocks (i.e., groups of multiple cells which are sometimesreferred to herein as “cell arrays”).

A system diagram of an embodiment of a redox flow battery energy storagesystem is illustrated in FIG. 1. The embodiment illustrated in FIG. 1utilizes a stack design for the redox flow battery which enables largescale applications to be implemented with common affordable batterycomponents. In applications where megawatts of electrical energy must bestored and discharged, e.g., a wind turbine farm or solar power plantcoupled to a power grid, the redox flow battery system illustrated inFIG. 1 can be expanded to the required capacity by increasing tank sizesand, expanded in terms of produced power by adding redox flow batterystack assemblies or cell blocks. Simply put, the amount of energy thatcan be stored is determined by the amount of electrolyte stored in thesystem. Thus, to store more energy, larger electrolyte storage tanks areused. To increase the output power, more redox flow battery cells and/orstack assemblies are added. Thus, the systems shown and described hereinprovide great flexibility in addressing a wide range of energy storagerequirements.

Referring to FIG. 1, the main components of the redox flow batterysystem include the redox flow battery stack assembly 10 through whichtwo electrolytes flow through porous electrodes 18, 20 which areseparated by a separator membrane 12. Reduction and oxidation reactionsthat can occur in the respective electrolytes cause electricity to flowthrough the reaction chamber which is captured by porous electrodes 18,20 and conducted to conductive surfaces 22, 24. In some embodiments flowchannels 14, 16 may be included in the redox flow battery stack assembly10 to reduce electrolyte flow restrictions through the stack. Includingsuch flow channels 14, 16 can be used to reduce electrolyte pressuredrops. In an embodiment, the flow channels 14, 16 may be incorporated sothat the electrolytes have sufficient interaction with the porouselectrodes 18, 20 to enable the required reduction and oxidationreactions to take place.

The conductive surfaces 22, 24 are coupled to conductors 42, 43 whichcomplete a circuit through either an electrical power source 45 (forcharging) or an electrical power load 46 (for discharge), which may beselected in single stack embodiments via an electrical switch 44. Thecathode electrolyte (“catholyte) and anode electrolyte (“anolyte”) arestored in electrolyte tanks 26, 28, and are pumped by pumps 30, 32 toprovide the input flows 34, 36 to the redox flow battery stack assembly10, with battery output flows 38, 40 returning to the electrolyte tanks26, 28. The redox flow battery stack assembly 10 is designed for reducecost by keeping the complexity and part count of the stack to a minimum.The redox flow battery stack assembly 10 is further designed to minimizeshunt current losses and maximizing reactant utilization.

The redox flow battery stack assembly 10 is configured to include anarray of independent battery cells, assembly frames as shown in FIGS. 2and 3. The independent battery cells are arranged so that electrolytereactant flows from one cell to the next within a stack layer 48 (seeFIG. 2). Multiple layers 48 of battery cells are stacked togetherconnected in series to form a stack assembly 10 as described below withreference to FIG. 7A. Further, the independent battery cells areconfigured to increase their electrochemical performance based upontheir location within the reactant flow path, thus resulting in a redoxflow battery assembly that has greater overall electrical storageperformance than possible with identical battery cells.

FIG. 2 illustrates a cross-section of an individual single cell layer 48within a redox flow battery stack assembly 10 as viewed from aperspective perpendicular to the plane of the electrodes 18, 20 and themembrane separator 12 (i.e., short axis of layer 48 is into and out ofthe page of FIG. 1). The illustrated cell layer 48 includes threeindependent cells 52, 54, 56, as an example embodiment; in otherembodiments each cell layer 48 may include fewer or more independentcells. In a preferred embodiment, the electrolyte reactant flows acrossall the cells in a cell layer 48 within the array (i.e., parallel to theimage surface of FIG. 2) in a cascading manner (i.e., one cell to thenext within a given layer). This multiple cell configuration within eachcell layer mitigates problems with shunt currents. To enhance overallefficiency and battery performance, the battery cells are configuredwith varying catalyst loadings, electrode tortuosity, chamber volumes,and/or membrane separator porosities or selectivity to handle thevariations in reactant concentration along the flow path, minimizeundesired reactions, and optimize coulombic and voltage efficiencies.For example, as illustrated in FIG. 2, in a three-cell redox flowbattery cell layer assembly 48, a first cell 52 near the reactant inletflows 34, 36 may be configured with structural and material propertiesto offer greater efficiency with the higher state of charge condition ofthe electrolyte at the input to the battery cell layer assembly. Asecond cell 54 may then be configured with structural and materialproperties to provide efficient operation with the intermediate state ofcharge condition of the electrolytes that will exist after theelectrolytes have passed through the first cell 52. The third cell 56may be configured with structural and internal properties to provideefficient operation with the relatively low state of charge conditionthat will exist in the electrolytes after they have reacted in the firstand second cells 52, 54. As described in more detail below, configuringthe redox flow battery cell layer assembly 48 in this manner providesefficient operation while enabling the battery to be assembled withlower-cost materials.

Some types of flow battery electrolytes operate more efficiently (i.e.,retaining and discharging electrical power with lower losses) when thefluids are heated to an optimum temperature. To take advantage of thischaracteristic, the redox flow battery cell layer assembly 48 may beconfigured with tubes 60, 62, 64, 66 or channels through which a heatingfluid can be circulated. Circulating a heating fluid around and/orwithin the battery stack assembly can keep the electrolytes at acontrolled temperature. By including heating fluid tubes 60, 62, 64, 66before and after each battery cell, the operating temperature of eachcell can be controlled individually so as to enable each cell to operateat a preferred or optimum temperature corresponding to the state ofcharge of electrolytes within the cell. The heating fluid tubes areoptional because in an embodiment the electrolytes may be preheatedwithin the tanks 26, 28, such as via a heat exchanger circulating aheating fluid so that the electrolytes enter the cell layers 48 at asufficient temperature for charging or discharging operations. Asdescribed more fully below, the heating fluid may draw thermal energyfrom waste heat generated by either the source of the charging power 45(e.g., from a generator cooling system) or the load 46 (e.g., from anequipment cooling system).

A conceptual build of a single cell of a cell section within the celllayer 48 of a flow battery stack is illustrated in FIGS. 3A and 3B. FIG.3A shows a cross-sectional view of a single layer of a single cellchamber 50 viewed from a perspective that is perpendicular to thecross-sectional perspectives in FIGS. 1 and 2. FIG. 3B shows an explodedview of a single cell 50 of an individual single cell layer. A bipolarframe formed from first and second planar structural members 80, 82provides the structural support for the redox flow battery stackassembly 10. The planar structural members 80, 82 may be made frompolyethylene, polypropylene or other materials which are resistant tothe mild acid of the electrolyte reactants. Between the planarstructural members 80, 82 is formed a cavity which contains the porouselectrode catalyst 18, 20 through which anolyte and catholyte reactantsflow 38, 40, respectively. The porous electrode may be made from aseparate carbon fiber felt material or may be part of the bipolar frameitself. The porous electrode catalysts 18, 20 may be made from carbonfelt material coated with a catalyst layer. In some implementations asurface catalyst layer may be lead (Pb), bismuth (Bi) or zirconiumcarbide (ZrC) to facilitate reduction-oxidation reactions with theelectrolytes while suppressing generation of hydrogen gas. Within eachplanar structural member 80, 82 may be provided cutouts or inserts forconductor surfaces 22, 24, as illustrated in FIG. 3B. The conductorsurfaces 22, 24 pass electric current from the porous electrodecatalysts to the exterior of the cell layer.

The anolyte and catholyte reactants are separated by a planar membraneseparator 12 which is suspended between the two planar structuralmembers 80, 82 by frame members 84, 86, 88, 90. It should be noted thatthe frame members 84, 86, 88, 90 maybe in the form of two exteriorframes as illustrated in FIG. 3B such that frame members 84 and 88 arepart of a single frame 84 and frame members 86 and 90 are part ofanother single frame 86. The membrane separator 12 allows ions totransport through the material while inhibiting bulk mixing of thereactants. As described more fully below with reference to FIGS.16A-16C, the membrane separator 12 may be made from different materialsso as to exhibit varying diffusion selectivity and electrical resistanceas appropriate for the expected state of charge within each batterycell.

At the reactant inlet of each battery cell 50, manifold holes 92, 94 maybe provided to direct the incoming electrolyte flows into the reactionarea of the cell 50. In an embodiment, the manifolds may include flowdirecting structures to cause proper mixing of the electrolytes as theyenter each reaction cell 50. Such flow directing structures may beconfigured to adjust or control the reactant flow in each cell 50 withinthe redox flow battery stack assembly 10 based upon the expected stateof charge and other fluid properties within each cell.

The planar structural members 80, 82, as well as separator frame members84, 86, 88, 90 may include passages through which heat exchanger fluidpipes 60, 62 can pass. Positioning optional heat exchanger fluid pipes60 within the cell input manifolds 92, 94 enables heat from the thermalfluid within the pipes to raise the temperature of the reactant flowsbefore the reactants enter the cell chamber. Similarly, positioning heatexchanger pipes 62 within the cell output manifolds 96, 98 enables thethermal fluid to extract heat from the electrolytes after the reactantsleave a final cell 56, thereby conserving thermal energy and enablingthe electrolytes to be returned to storage tanks at a coolertemperature. In a preferred embodiment the thermal fluid is heated to atemperature of about 40 to 65° C. for Fe/Cr reactants.

A redox flow battery stack assembly 10 may be formed by stacking layers48 in series to form a battery stack. In this battery stack assembly theconductive surfaces 22, 24 provide the electrical connectivity betweencells in each stack cell layer as described below with reference to FIG.7A.

The planar structural members 80, 82 which form the bipolar frame may beelectrically conductive throughout their area, or may be made in such away that only the conductive surfaces 22, 24 immediately adjacent to theelectrochemically active portion of the cell 50 are electricallyconductive, as illustrated in FIG. 3B. In the latter embodiment, thearea around the conductive surfaces 22, 24 may be electricallyinsulating. Electrically insulating the areas around conductive surfaces22, 24 allows for discrete control and monitoring of the current orpotential of each type of cell in the redox flow battery stack assembly10.

To form each cell layer 48 as illustrated in FIG. 2, multiple cells 50as illustrated in FIGS. 3A and 3B are fluidically connected to form acascade of cells within a single layer. Thus, the cell output manifolds96, 98 of one cell line up with the cell input manifolds 92, 94 of thenext cell within the cell layer 48 so the electrolyte flows from onecell to the next within each cell layer.

In the redox flow battery system of the various embodiments the cellscan be replaceable and recyclable. Since the materials of constructionare primarily plastics (e.g., polypropylene or polyethylene), carbonfiber felts, and carbon fiber electrodes, the cells contain no heavymetals or toxins that could pose an environmental impact. Further, thereactants, such as Fe/Cr, are no more toxic or dangerous than batteryacid. Thus, the redox flow battery system of the various embodiments areideal for providing the energy storage capacity required for renewableenergy systems in a distributed fashion close to the population and loadcenters.

As explained more fully below with reference to FIG. 8, the porousseparator 12 may be fused to a dense or partially dense state around theedges to prevent electrolyte reactants from seeping through the sealededge regions. This reduces reactant mixing and leakage out of the redoxflow battery stack assembly 10. Electrolyte reactant mixing through theporous membrane separator 12 is minimized because the concentration ofthe reactants on both sides of the membrane separator 12 areapproximately the same, as the described below, with similar iondensities, thereby eliminating concentration gradients and reducingosmotic pressure across the membrane separator 12.

A variety of reactants and catalysts may be used in the redox flowbattery system. A preferred embodiment set of electrolyte reactants isbased upon the iron and chromium reactions illustrated in FIG. 4. Thereactants in the Fe/Cr redox flow battery system stores energy inFeCl₃(Fe³⁺) in the catholyte, which reacts at the positive electrode,and CrCl₂(Cr²⁺) in the anolyte, which reacts at the negative electrodewithin cells of the battery.

An undesirable non-faradic electron transfer reaction can occur betweenFe³⁺ and Cr²⁺ if these ions come into proximity to one another.Therefore, to maintain a high level of coulombic efficiency, electrolytecross-mixing within a Fe/Cr redox flow battery stack should beminimized. One way to minimize electrolyte cross-mixing is to use ahighly selective membrane separator 12 such as Nafion®-117 ion-exchangemembrane (DuPont, USA). A disadvantage of highly-selective membraneseparators is that they have low ionic conductivity which results inlower voltage efficiency within the redox flow battery stack.Additionally, ion-exchange membranes are expensive, with a price in theneighborhood of $500/m². Since the DC energy storage efficiency of aredox flow battery is the product of coulombic and voltage efficiencies,an optimization tradeoff exists.

A particular embodiment of the Fe/Cr system is what is known as themixed reactant system where FeCl₂ (Fe²⁺) is added to the anolyte andCrCl₃ (Cr³⁺) is added to the catholyte, as described in U.S. Pat. No.4,543,302, the entire contents of which are incorporated herein byreference. An advantage of the mixed reactant system is that thedischarged anolyte and discharged catholyte are identical. Furthermore,when the total concentration of Fe in the anolyte is the same as thecatholyte, and the total concentration of Cr in the catholyte is thesame as the anolyte, the concentration gradients across the membraneseparators 12 are eliminated. In this way the driving force forcross-mixing between anolyte and catholyte is reduced. When the drivingforce for cross-mixing is reduced less selective membrane separators maybe used, thereby providing lower ionic resistance and lower systemcosts. Examples of less-selective membrane separators includemicroporous membrane separators manufactured by Celgard LLC, andmembrane separators made by Daramic LLC, both of which cost in theneighborhood of $5 to 10/m². By optimizing the cell characteristics forthe reactant state of charge and completing the charge or discharge inone pass, the embodiments described herein provide suitably highefficiency in a redox flow battery stack comprised of materials that areapproximately two orders of magnitude lower cost than in conventionalredox flow battery designs.

In both the unmixed and mixed reactant embodiments, the reactants aredissolved in HCl, which is typically about 1-3 M concentration. Theelectrocatalyst, which may be a combination of Pb, Bi and Au or ZrC, isprovided at the negative electrode to improve the rate of reaction ofrecharging when Cr³⁺ in the anolyte is reduced to Cr²⁺, thereby reducingor eliminating hydrogen evolution. Hydrogen evolution is undesirable asit unbalances the anolyte from the catholyte and is a competing reactionto Cr³⁺ reduction leading to a reduction in coulombic efficiency.

The cell, cell layer and redox flow battery stack designs describedherein can be used with other reactant combinations that includereactants dissolved in an electrolyte. One example is a stack containingthe vanadium reactants V(II)/V(III) or V²⁺/V³⁺ at the negative electrode(anolyte) and V(IV)/V(V) or V⁴⁺/V⁵⁺ at the positive electrode(catholyte). The anolyte and catholyte reactants in such a system aredissolved in sulfuric acid. This type of battery is often called theall-vanadium battery because both the anolyte and catholyte containvanadium species. Other combinations of reactants in a flow battery thatcan utilize the embodiment cell and stack designs include Sn(anolyte)/Fe (catholyte), Mn (anolyte)/Fe (catholyte), V (anolyte)/Ce(catholyte), V (anolyte)/Br₂ (catholyte), Fe (anolyte)/Br₂ (catholyte),and S (anolyte)/Br₂ (catholyte). In each of these example chemistries,the reactants are present as dissolved ionic species in theelectrolytes, which permits the use of battery cell and stack designs inwhich electrolyte flow through a plurality of battery cells series alongthe flow path (i.e., cascade flow), with the cells and having differentphysical properties along the flow path (cell size, type of membrane orseparator, type and amount of catalyst). A further example of a workableredox flow battery chemistry and system is provided in U.S. Pat. No.6,475,661, the entire contents of which are incorporated herein byreference.

A number of cell chambers are formed in each bipolar frame in a redoxflow battery stack array. FIG. 2 depicts a 1×3 array, but anycombination is possible, e.g., a 2×2 or a 1×4 array. As described above,the electrolyte reactant flows from one cell 52, 54, 56 to the next in acascade arrangement. This cascade flow means that a cell 52 closest tothe inlet will see higher reactant concentrations than downstream cells54, 56 in the discharge mode. For example, for the Fe/Cr system indischarge mode, the Fe³⁺ and Cr⁺² species are the relevant ionconcentrations as shown in FIG. 4. This cascading of battery cellsarrangement provides the advantage of limiting shunt currents andimproving overall reactant utilization. Shunt currents are formed due toshort circuiting within the liquid reactants. It is thereforeadvantageous to form long conductive paths between one cell and thenext, as well as limit the stack voltage. The various embodimentsaccomplish both objectives by flowing the reactants across multiplecells within the same layer. This cascade flow regime also improvesreactant utilization compared to that of a single cell per layer stackarrangement. Improving reactant utilization helps to improve theroundtrip DC efficiency of the redox flow battery stack assembly 10 andreduces or eliminates the need to re-circulate the reactants.Recirculation can be disadvantageous because it may involve more pumpingpower per kW or stored capacity, which increases parasitic losses.

Due to the variation in reactant ion concentrations as the reactantsflow through the various cells in each layer, the amounts of catalyticcoating may be varied to match the state of charge condition in each ofthe respective cells. Additionally, the catalytic coating formationsapplied to the porous electrodes 18, 20 may be varied in formulation(e.g., varying amounts of zirconia or bismuth compounds) to better matchthe state of charge condition in each cell. For example, typically thecell with the lower reactant concentrations will require a highercatalyst loading on the porous electrodes to achieve optimumperformance.

The various embodiments include a unique redox flow battery stackconfiguration that includes multiple independent cells within a flowpath as illustrated in FIG. 2, with each independent cell configured interms of size, shape, electrode materials and membrane separator layer12 material for optimum average performance with the state-of-charge ofreactants within each cell. FIG. 5 summarizes some of the designconfiguration parameters that can be controlled and the manner in whichthe parameters are varied along the reactant flow path in order tomaximize electrical performance of each independent cell in the redoxflow battery stack assembly 10. As illustrated in design trend line 112,some design parameters—illustrated as Group A parameters—may bedecreased from one end of a cell layer 48 to the other to configure thebattery design so that the values decrease from reactant inlet to outletfrom the cell layer in discharge mode and increase from reactant inletto outlet from the cell layer in charging mode.

As illustrated in design trend line 116, other designparameters—illustrated as Group B parameters—may be increased from oneend of a cell layer 48 to the other to configure the battery design sothat the values increase from reactant inlet to outlet from the celllayer in discharge mode and decrease from reactant inlet to outlet fromthe cell layer in charging mode. As illustrated in FIG. 5, the designparameters that may be varied to configure battery cell designsaccording to design trend line 112 include: membrane selectivity; chargecatalyst loading; charge catalyst activity; temperature (when optimizingcharging); chamber volume (when optimizing charging); mass transport(when optimizing charging). The design parameters that may be varied toconfigure battery cell designs according to design trend line 116include: ionic conductivity; discharge catalyst loading; dischargecatalyst activity; temperature (when optimizing discharging); chambervolume (when optimizing discharging); mass transport (when optimizingdischarging).

For example, as described above, the discharge catalyst loading anddischarge catalyst activity (both Group B design parameters) may beincreased in each cell along the flow path of redox flow battery stackassembly 10 from inlet to outlet in the discharge mode and decreased ineach cell along the flow path of redox flow battery stack assembly 10from inlet to outlet in the charge mode to compensate for decreasingreactant concentrations, as indicated by the design trend line 116.

Similarly, the charge catalyst loading and charge catalyst activity(both Group A design parameters) may be decreased in each cell along theflow path of redox flow battery stack assembly 10 from inlet to outletin the discharge mode and increased in each cell along the flow path ofredox flow battery stack assembly 10 from inlet to outlet in the chargemode to compensate for decreasing reactant concentrations, as indicatedby the design trend line 112. The specific catalyst loading andcatalysts activity implemented within each cell along the flow path canbe determined using the design trend line 116 with respect todischarging, trend line 112 with respect to charging, and the number ofcells in the path.

Using the design trend lines 112, 116 illustrated in FIG. 5, in someredox flow battery embodiments provide improved electrochemicalperformance by optimizing design parameters, such as the charge anddischarge catalyst loadings and/or catalyst activities, in each layer ineither direction through the battery stack, and flowing the reactantsthrough the battery stack in one direction for discharging and in theopposite direction for charging. In some embodiments, such as describedbelow with reference to FIGS. 14-15C, the reactants are directed throughthe redox flow battery in one direction in the charging mode and in theopposite direction in the discharging mode. In other embodiments, suchas described below with reference to FIGS. 13A-13D, separate chargingredox flow battery stacks are provided for charging and for dischargingso the reactants flow in a single direction consistent with the cellconfiguration. In a third embodiment described below with reference toFIG. 1, electrolyte reactants flow through the redox flow battery stackin a single direction for both charging and discharging with the batterycells configured as a compromise between charging and discharging (e.g.,preferentially configured for charging or discharging) so that thesystem can switch between charging and discharging modes very quicklysimply by electrically disconnecting the redox flow battery stackassembly 10 from the charging power source (e.g., with an electricalswitch) and connecting the stack to the load, or vice versa.

Similarly, the various embodiments may control the temperature ofreactants as they flow through the redox flow battery stack dependingupon whether the stack is charging or discharging. FIG. 5 illustrates indesign curves 112 and 116 how the temperature may be controlled in anembodiment along the flow path through the redox flow battery cell layer48 and stack assembly 10. For the chosen optimized half-cycle, at eachcell along the reactant flow path in the discharge mode the temperatureis increased so the cell closest to the outlet, which will have thelowest concentration of reactants, runs at a higher temperature than thecell closest to the inlet. The design curve to employ a given redox flowbattery cell layer 48 and stack assembly 10 may be based on whether agreater improvement in battery efficiency is achieved by optimizing thedischarge reactions or the charge reactions. In the Fe/Cr system, theanolyte charge reaction has the most limited reaction rates so designtrend line 112 would be selected for the temperature profile designparameter. As with catalyst loading and catalysts activity, the redoxflow battery cell layer 48 and stack assembly 10 can be configured sothat reactants flow in one direction for charging and in the otherdirection for discharging, or as two separate redox flow battery stackscan be used with one configured for charging and the other configuredfor discharging.

In a similar manner, the various embodiments improve electrochemicalperformance by configuring the redox flow battery stack assembly 10 sothat the reactant mass transport rate varies from cell to cell along theflow path. FIG. 5 also illustrates in design curve 116 how cells areconfigured so that the reactant mass transport rate increases in eachcell along the flow path from inlet to outlet in the discharge mode, anddecreases in each cell along the flow path from the inlet to the outletwhile in the charging mode. The mass transport rate may be increased bydecreasing the physical dimensions of each cell and selecting electrodecatalyst materials to vary the electrode porosity. Thus, an embodimentredox flow battery stack assembly 10 may have a restricted flow area atone end and a more open and less constricted flow area at the other end,with the reactant mass transport rate increasing in each cell along thereactant flow path when operated in the discharge mode, and decreasingin each cell along the reactant flow path when operated in the chargingmode.

In a similar manner, embodiment redox flow battery cells may beconfigured with different membrane separator 12 materials along thereactant flow path. FIG. 5 illustrates in the design curve 112 how themembrane separator 12 selectivity (i.e., the degree to which thereactants are restricted from moving through the separator) in each cellis varied along the reactant flow path. Cells near the inlet to theredox flow battery stack assembly 10 in the discharge mode willexperience a high concentration of reactants (e.g., Cr²⁺ and Fe³⁺), andthus mixing of the reactants through the membrane separator 12 willresult in greater losses of stored energy than is the case in cells nearthe outlet of the assembly. Therefore, the various embodiments achievegreater electrical charge/discharge efficiency by limiting the diffusionof reactants through the membrane separator 12 near the battery inlet.On the other hand, membrane separator materials which have high membraneselectivity typically also exhibit high ohmic losses (i.e., electricalresistance), which increases energy losses through the battery due tointernal resistance. The countervailing properties result in the designcurve 112 shown in graph 110 in FIG. 5 used to select separatormaterials depending upon the number of cells in the reactant flow path.

Thus, in an embodiment redox flow battery stack assembly 10 may includecells at one end of the flow path having membrane separators 12 madefrom a material with high membrane selectivity at the cost of greaterohmic losses, while cells at the other end of the flow path will havemembrane separators 12 made from a material with lower ohmic losses.This design approach works because the driving force for cross mixing isgreatly diminished due to the low concentrations ofspontaneously-reacting active species at the outlet end in the dischargemode and at the inlet end in the charge mode. In the case of an Fe/Crredox flow battery (FIG. 4) the concentration of Cr²+ and Fe³+ speciesare at a minimum at the outlet end in the discharge mode and at theinlet in the charge mode.

As mentioned above, the particular design configuration of each cellwithin a particular redox flow battery stack assembly 10 may bedetermined by applying the design trend lines illustrated in FIG. 5 tothe number of cells along the reactant flow path within the assembly.Cells may be configured with design parameters selected for the averageelectrolyte concentration expected within each cell, which may provide astair step approximation of the design trend lines illustrated in FIG.5. By increasing the number of independent cells along the reactant flowpath, the cell design parameters can better match the design trendlines. However, increasing the number of independent cells may adddesign complexity which may increase system costs. Thus, the number ofcells and the design configurations applied to each cell will be variedbased upon the design goals and performance requirements of particularimplementations.

By varying the design configurations of independent cells along thereactant flow path through the redox flow battery cell layer 48 andstack assembly 10 the various embodiments are able to achievesignificant charging/discharging performance improvements overconventional redox flow battery designs. This performance improvement isillustrated in FIG. 6 which shows the polarization curve 122 (outputvoltage as a function of output current) of a convention redox flowbattery that does not include the embodiment enhancements. This poorperformance curve 122 falls well below the ideal performance curve 120which may be approached by the embodiment redox flow battery designsimplementing the embodiment configurations described above.

By forming the conducting regions (e.g., conductive surfaces 22, 24)only on the active areas of the bipolar frame as illustrated in FIG. 3B,the redox flow battery stack assembly 10 can be made quite flexible. Aplurality of cell layers 140-148 can be formed into a stack byassembling the layers one on top of another so that the conductivesurfaces 22, 24 of each cell chamber in the cell layers 48 (one of whichis illustrated in FIGS. 3A and 3B) connect electrically in series, andturning the stack into a vertical orientation as illustrated in FIG. 7A.Positioning the redox flow battery stack assembly 10 in a verticalorientation, so that one cell 52 within a layer is on the bottom and theopposite cell 56 is on the top, aids in venting any hydrogen that may beformed during charging or discharging reactions. Separate terminals maybe coupled to the exterior conductive surfaces 22, 24 as depicted inFIG. 7A in order to connect the battery to a load. Coupling a number ofterminals in the manner illustrated in FIG. 7A, can enable separatemonitoring of each of the cell columns (i.e., the cells connectedelectrically in series across a stack) along the flow path which canenable better control of the stack. By monitoring the voltage acrosseach of the cell columns along the vertical length, the precise state ofcharge can be determined. Depending on the power demand placed upon theredox flow battery stack assembly 10, the battery can be fully utilizedfor peak demand or just partially utilized when the demand is small.Each stack can be individually controlled in terms of current loading toprovide for longer life or higher efficiency.

FIG. 7B illustrates an embodiment of a redox flow battery stack assembly10 in which the stack is formed by stacking cell layers 48 which areformed in unibody frames 48 a, 48 b, 48 c. As illustrated in FIG. 7B, inthis embodiment, individual cells are formed within frames that span thelength of the cell layer. As mentioned above, the design parameters ofeach cell 1 52 a, 52 b, 52 c are configured according to the chargestate of reactants in those cells, and thus may be different from thedesign parameters of each cell 3 56 a, 56 b, 56 c within the cell layers48 a, 48 b, 48 c of the stack 10.

Instead of assembling cells within a unibody frame for each cell layer,each cell may be assembled within cell frames 52 a-56 c in an embodimentillustrated in FIG. 7C, as well as FIG. 3B. In this embodiment, theredox flow battery stack can be assembled by fitting cells 52 a, 54 a,56 a (e.g., electrode 18, membrane separator 12, and electrode 20 ofFIG. 3B) into cell frames (e.g., frames 84 and 86 of FIG. 3B) and thenstacking the like design framed cells (e.g., all cell 52's in aconfiguration like FIG. 3A) with interleaved bipolar plates (e.g.,conductive areas 22 within frame 82 of FIG. 3B) to form cell columns 72,74, 76, which are then fit together to complete the stack 10.

As mentioned above, one source of losses in a redox flow battery is dueto mixing or leakage of reactants along the edges of the membraneseparator 12. As illustrated in FIG. 8, such losses may be eliminated bysealing the membrane separator material edges 160, 162. Such edgesealing may be accomplished by fusing the material by heating it to anelevated temperature while compressing it, such as with an iron or vise.Alternatively, gaskets can be used around the periphery of each cellchamber for sealing.

As mentioned above, the performance of a redox flow battery stackassembly 10 can be enhanced by heating the reactants to optimumtemperatures at various stages within the battery flow path. Variousembodiments accomplish such heating by using waste heat or alternativeenergy heat sources, thereby enhancing electrical performance whilereducing parasitic losses. The various embodiments have a number ofuseful applications in energy generation applications as well asindustrial applications which use electrical power and generate wasteheat (e.g., heat sinks from air-conditioning and equipment coolingsystems). As discussed in the embodiments below, alternative energysources such as wind turbines and solar panels require cooling toenhance performance and prevent mechanical breakdown. Larger energystorage systems using the Fe/Cr redox flow battery technology can bethermally integrated with wind turbine farms and photovoltaic solarfarms as illustrated in FIGS. 9-11 to use low grade waste heat in acomplimentary fashion. For example, a 1 MWh/4 MWh redox flow batterysystem can be thermally and electrically connected to a small number ofwind turbines.

Integrating a wind turbine system with a redox flow battery systemprovides a renewable power generation system which can operate moreefficiently and economically than a wind turbine farm that does not haveenergy storage capacity. Such a system can store power whenever the windis blowing, and meet the power demands of the electrical power gridregardless of the current wind conditions. This enables a windturbine/redox flow battery system to meet utility contractualobligations to provide consistent power to the electrical power grid,thereby avoiding economic penalties for failing to supply contractedpower levels during times of little or no wind. Additionally, the systemallows electrical power to be supplied to the power grid during periodsof peak demand, enabling the system owner to sell electrical power atthe most favorable rates regardless of when peak winds occur.

An embodiment energy generation and storage system combining a windturbine farm 170 with a redox flow battery is illustrated in FIG. 9. Asmentioned above, wind turbines generally require a cooling water systemto ensure that the mechanical systems operate within design temperatureranges as described. The cooling water circulated through the turbinestructures 170 can be used as a heating fluid 174 for the redox flowbattery system 172. Thus, the waste heat generated by mechanicalfriction in the wind turbines can be partially recovered in terms ofoverall energy output performance by using that energy to maintain thereactants in the flow battery system 172 at an optimum operatingtemperature. The electrical power 176 generated by the wind turbine farm170, which is often generated at times that do not correspond with peakpower demand, can be stored in the redox flow battery system 172. Thestored electrical power 178 can then be used to provide dispatchablepeak power to the grid in response to demand, such as during times ofpeak power demand. FIG. 9 depicts a 1 MW flow battery system integratedwith three 600 kW wind turbines. Thus, a redox flow battery stackassembly 10 provides an ideal solution to the energy storage challengeof inconsistent energy generators while utilizing the waste heatrequired for cooling such alternative energy systems.

Similar to the wind turbine/redox flow battery system described abovewith reference to FIG. 10, integrating a solar energy conversion systemwith a redox flow battery system provides a renewable power generationsystem which can operate more efficiently and economically than a solargeneration system that does not have energy storage capacity. Such asystem can charge the battery to store power whenever the sun isshining, and meet the power demands of the electrical power gridregardless of the time of day or weather conditions. This enables such asolar generator/redox flow battery system to meet utility contractualobligations for providing consistent power to the electrical power grid,thereby avoiding economic penalties for failing to supply contractedpower levels during times of cloudy weather or at night. Additionally,the system allows electrical power to be supplied to the power gridduring periods of peak demand, enabling the system owner to sellelectrical power at the most favorable rates regardless of the time ofday or weather.

A solar energy conversion system, such as a photovoltaic (PV) array,concentrating photovoltaic (CPV) array, a solar thermal energy powerplant, or a solar hot water system, can be thermally and electricallyintegrated with the redox flow battery system to provide a moreeconomical and efficient renewable energy generation system 180, 190 asillustrated in FIGS. 10 and 11. A solar collector 183 may generateelectricity as well as capture solar heat energy. In a solar electricgeneration system water may be circulated through or under thephotovoltaic panels to maintain the photovoltaic cells within designoperating temperatures. The heat energy received by the solar collector183 may be stored in a thermal storage tank 182. As described above, theFe/Cr redox flow battery operates at optimum efficiency at temperaturesin the range of about 40 to 65° C. Heating fluids (e.g., water) from thethermal storage tank 182 can be use to provide the required heat energyto maintain this temperature in the redox flow battery stack assembly 10without incurring costly parasitic losses or additional operating costs(and greenhouse gas emissions), as would be the case in an electric orgas fired heating system. Solar collectors and thermal storage systemsrepresent a very mature technology, particularly in residential markets.In an embodiment, the electrolyte itself can be the working fluid in athermosiphon hot water system.

Thermally integrating a solar thermal energy collection system with aredox flow battery system can be accomplished in at least twoconfigurations. In a first configuration illustrated in FIG. 10, thesolar collector 183 and thermal storage tank 182 are designed to holdthe electrolyte reactant, which is a solution of HCl in the case of theFe/Cr system. In this configuration the reactant is raised to atemperature of about 40 to 65° C. in the solar collector 183 and thermalstorage tank 182, so that reactant flowing out of the thermal storagetank 182 is pumped (via pump 186) directly into the redox flow batterystack assembly 10 where it takes part in the electrochemical reactions.Reactants exiting the redox flow battery stack assembly 10 are returnedto the thermal storage tank 182 for reheating. Alternatively, a closedloop heating fluid can be used in the solar collector 183 with heattransferred from the heating fluid to the electrolyte stored in thethermal storage tank 182 in a heat exchanger within the tank as in theclosed loop solar hot water system embodiment.

In a third configuration illustrated in FIG. 11, hot water (or anotherfluid) produced by solar collector 183 may be used as the heating fluidstored in the thermal storage tank 182 which is pumped into and aroundthe redox flow battery stack assembly 10, such as through heat exchangetubes. In this configuration, the heating fluid from the thermal storagetank 182 does not mix with the electrolyte reactants.

Thermally integrating a solar collector or solar energy conversionsystem with a redox flow battery system can use either pump circulationas illustrated in FIG. 10, or natural circulation (thermo siphon) asillustrated in FIG. 11. Pumping the heating fluid through the redox flowbattery stack assembly 10 (as the reactants or as a heating fluidflowing through heat exchanger pipes) can provide improved thermalperformance, but at the cost of parasitic losses from the power consumedby the pump 186. In a natural circulation configuration as illustratedin FIG. 11, the buoyancy of the heated water or reactant is used tocause the fluid to circulate through the redox flow battery stackassembly 10 without the need for a pump. The hot water rises from thetop of the thermal storage tank 182 and passes through the redox flowbattery stack assembly 10 where it is cooled, increasing its density.With no moving parts or fossil fuels required the solar heated naturalcirculation configuration does not suffer parasitic losses which wouldlimit the overall roundtrip efficiency of the energy storage system. Thenatural circulation configuration avoids parasitic losses associatedwith running cooling pumps and provides a very simple system with asingle working fluid, which may well be a good solution for smallersystems because of the constrained tank volume. On the other hand,enabling natural circulation flow may require configuration compromises,such as locating the redox flow battery stack assembly 10 above thethermal storage tank 182, such as on the roof of a building in closeproximity to the solar collector 183 or thermal storage tank 182.

The thermosiphon solar heating system operates in closed loopconfiguration for both embodiments illustrated in FIGS. 10 and 11. Thethermal storage tank 182 can be of a manageable size for larger energystorage systems because it is just circulating a high heat capacityfluid (e.g., water) when used to maintain the temperature of the redoxflow battery stack assembly 10.

The table in FIG. 12 exemplifies sizing parameters for commerciallyavailable solar hot water systems that would be suitable for use withvarious configurations of redox flow battery systems.

Thermal integration of a redox flow battery system with conventionalpower generation systems, such as nuclear and coal-fired power plants,can provide significant energy and economic efficiencies since suchsystems generate a large amount of low grade waste heat. As describedabove, thermally integrating the redox flow battery system with sourcesof waste heat improves the battery operating efficiency without theexpense or parasitic losses of electrical or fossil fuel heaters.Electrically integrating a redox flow battery energy storage system withconventional power generation systems also provides significant economicadvantages since the battery system can enable base-loaded power plantsto accommodate grid support (ancillary services) or peak power demandswithout varying their output. As is well known, nuclear and coal-firedpower plants operate most efficiently and economically when run atconstant power levels. Peak power demands can be met by charging theredox flow battery energy storage system during periods of reduceddemand (e.g., off-peak hours in the late evening) and then augmentingthe electrical output of the power plant with electricity drawn from thebattery system during periods of peak power demand. Such a combinedpower plant/energy storage system can be economically advantageous sinceelectrical power can be generated in the most economical manner (i.e.,at constant output 24 hours per day) but sold at times of peak demandwhen electricity prices are greatest. Further economic benefits can beobtained by adding a redox flow battery energy storage system to anestablished conventional power plant to meet growing demands for peakpower without building additional power plants. The sizing flexibilityof redox flow battery systems, in which energy storage capacity can beincreased simply by increasing the size or number of reactant storagetanks, means the economic advantages of adding a flow battery storagesystem to a conventional power plant can be obtained without having toinvest in a system sized for future demands.

Geothermal energy can also be used to heat the reactant storage tanks.This approach could provide a stable system with a large amount ofthermal inertia. Low grade geothermal energy can be used to provide heatto the redox flow battery stack assembly 10 or to the reactant storagetanks. In this embodiment heat is obtained from geothermal energy deepwithin the Earth which can be conveyed by a thermal fluid around thereactant storage tanks and/or through a heat exchanger before and afterthe battery stack.

The redox flow battery storage system does not necessarily need to beplaced in close proximity to the power generation system. For example,if there is a low cost source of waste heat from an industrial processor a solar array (PV or CPV) used to a building, it may be economicallyand efficiently advantageous to locate a redox flow battery within ornear the building in which the process is accomplished or the solararray located. In this manner, the waste heat from the industrialprocess or on-site power or thermal energy generation can be used toenhance the battery efficiency, while the energy storage capacity of thebattery is used to meet peak power demands or enable purchasingelectrical power during off-peak hours when electricity rates are lower.Thus, if the industrial process uses large amounts of electricity,thermally and electrically integrating the process with a redox flowbattery system can meet the process's demand for electrical power whileelectricity is purchased to charge the battery system during off-peakhours when electricity rates are lower. This type of implementation mayreduce cooling costs for the industrial process over periods when theelectricity rates are high, thus providing further cost savings.

All the previously mentioned low grade heat sources can also be appliedto heating the reactant tanks instead of or in addition to heating theredox flow battery stack assembly 10. Heating the reactant tanks enablesthe system to respond very quickly to load changes without any thermalmanagement problems because the reactant fluid is constantly maintainedat the operating temperature ready to be utilized in the flow battery.Costs and complexities of heating and insulating the reactant storagetanks may be offset by the cost advantages of simplifying the redox flowbattery stack design because this approach eliminates the need for heatexchanger elements within the battery stack assembly. Further, combiningthese alternative embodiments, such as heating storage tanks andproviding heat exchangers within the stack may provide the optimumdesign approach for providing clean, low cost and reliable heat to theredox flow battery.

Four additional example system embodiments of the redox flow batterysystem for use in battery energy storage systems (BESS) are illustratedin FIGS. 13A-13D. These example embodiments are intended to illustratehow various battery system components can be assembled into energygeneration systems in order to provide stored electrical power todifferent applications.

In a first example embodiment illustrated in FIG. 13A, a redox flowbattery energy storage system configuration different from the systemshown in FIG. 1 is used to provide a reliable source of direct current(DC) electrical power 200 that is fully isolated from fluctuations andsurges of the utility power grid. This embodiment system uses dual redoxflow battery stacks 210, 212 to enable simultaneous charging anddischarging operations. In this embodiment system 200, electrical powermay be received from a conventional electric utility grid 202, from anon-site renewable energy source 204, such as a wind turbine farm orsolar photovoltaic panels, and/or from an onsite distributed generator(DG) 205, such as a fuel cell 352, a propane generator (not shown), anatural gas micro-turbine (not shown), or a diesel generator set (notshown). Power from the grid 202, some renewable energy sources 204 ordistributed generator 205 may be rectified to generate DC power in apower conversion system 208, while DC power from a fuel cell 352,photovoltaic solar source 183 (see FIG. 10), or other DC generator willnot require the rectifier. The received DC power may be provided to afirst redox flow battery stack 210 which is configured for and dedicatedto charging the redox flow battery reactants. As DC power is provided tothe first (charging) redox flow battery stack 210, anolyte and catholytereactants are pumped into the charging redox flow battery stack 210 bypumps 226, 228. The DC power causes the anolyte and catholyte reactantsto be charged by converting Fe⁺² ions to the Fe⁺³ state and Cr⁺³ ions tothe Cr⁺² state (see FIG. 4). Such charged reactants emerge from thefirst redox flow battery stack 210 in outlet flows 230, 232 which aredirected to the anolyte tank 214 and catholyte tank 216, respectively.Thus, electrical power is stored in the Fe⁺³ and Cr⁺² electrolyteconcentrations in the storage tanks 214, 216.

Electrical power is generated from the chemical energy stored in theelectrolytes in a second (discharging) redox flow battery stack 212.Electrolyte from the storage tanks 214, 216 is directed to the secondredox flow battery stack 212 via inlet flows 218, 220. Within the secondredox flow battery stack 212, electricity is generated by convertingFe⁺³ ions to the Fe⁺² state and Cr⁺² ions to the Cr⁺³ state (see FIG.4). The generated electrical output 234 is provided to a DC load 206.

Reactants flowing out of the second redox flow battery stack 212(outflows 222, 224) may be pumped into the first redox flow batterystack 210 for recharging, thereby providing a single charging anddischarging loop. Since the electricity provided to the DC load 206 isgenerated from electrolytes in the second redox flow battery stack 212,the output current is completely isolated from the electrical sources ofcharging power, enabling the output power to reliably follow the DC loadwithout power spikes or power drops. This arrangement ensures powervariations from the grid, on-site renewable energy generators, oron-site distributed generators do not disrupt power to the DC load 206.Conversely, the power fluctuations associated with a large and widelyvarying load, such as an electric vehicle charging station or industrialbatch process (e.g., a mixer), remain isolated from the utility grid 202and other energy sources. This is beneficial to utilities as it reducesstress on the grid and also is beneficial to charge station owners as itcircumvents large power demand charges. The unique characteristics ofthe redox flow battery system also enables DC□DC conversion to beaccomplished with high overall system efficiency by a suitable choice ofthe number of cells connected in series within each stack to achieve V1in the charge stack and V2 in the discharge stack. Also, the facilityowner can choose when to charge the system so as to select the lowestcost electricity in order to maximize gross profit margins.

As described above, electrical efficiencies of the first and secondredox flow battery stacks 210, 212 can be enhanced by heating thereactants to an elevated temperature, such as about 40 to 65° C., usingon-site waste heat from equipment or facility cooling systems orgeothermal heating systems 236. As described above, a heating fluid fromwaste heat recovery systems, solar hot water system, or geothermalheating systems 236 may be provided to a heat exchanger within the redoxflow battery stacks 210, 212 (as illustrated in flow 238) and/or to heatthe reactant storage tanks 214, 216 (as illustrated in flow 240).

The embodiment illustrated in FIG. 13A provides a source of power forthe load 206 which is electrically isolated from the variability of theinput power, such as the utility grid 202, on-site renewable energysource 204 or onsite distributed generator 205. If the design goal is tosimply provide electrical isolation, the system 200 may use smallelectrolyte reactant tanks 214, 216 (e.g., sufficient tankage to acceptthermal expansion of the electrolyte and to store the electrolytes whenthe redox flow battery stack assemblies 210, 212 are drained formaintenance). This is because the reactants can be charged at the samerate they are discharged. However, by employing larger electrolytereactant tanks 214, 216 the system can also serve as a back up powersupply to provide electrical power to the load 206 when input power(e.g., from a utility grid 202) is not available.

A particularly attractive application for the Fe/Cr redox flow batterysystem 200 embodiment illustrated in FIG. 13A is as a powerisolator/uninterruptible power supply for a data center. Data centersrequire a particularly high quality of DC power and also emit a largeamount of waste heat. Presently, lead-acid battery based UninterruptiblePower Supplies (UPS) are used in data centers to ensure high-quality DCpower as well as short-duration back-up power. Heat exacerbates thepositive-grid corrosion and sulfation failure mechanisms of lead-acidbatteries necessitating operating such UPS systems in atemperature-controlled environment. In contrast to lead-acid batteryUPS, a Fe/Cr redox flow battery system of the embodiment illustrated inFIG. 13A can provide a reliable power supply while utilizing the wasteheat of the data center to improve overall system efficiencies, therebyproviding substantial advantages over lead-acid based UPS.

As described above with reference to FIG. 2 and FIG. 5, the first andsecond redox flow battery stacks 210, 212 of FIG. 13A are configured tohave multiple cells in each cell layer of the stack, with the cellswithin each cell layer configured to design parameters, such as matchcatalyst loading, catalyst activity, temperature, reactant masstransport rate and separator membrane selectivity, to the electrolyteconcentration expected in each cell along the reactant flow path. In theFe/Cr redox flow battery embodiment illustrated in FIG. 13A, the firstredox flow battery stack 210 is configured for charging so chargecatalyst loading, charge catalyst activity, temperature, mass transportrate, and separator membrane selectivity increase in succeeding cellsalong the flow path from inlet to outlet. In contrast, the second redoxflow battery stack 212 is configured for discharging so dischargecatalyst loading, discharge catalyst activity, temperature and masstransport rate increase, and separator membrane selectivity decrease insucceeding cells along the flow path from inlet to outlet.

In a second example embodiment illustrated in FIG. 13B, a redox flowbattery energy storage system can be used to provide the electricalpower for an electric vehicle (EV) or plug-in hybrid electric vehicle(PHEV) charging station 250. This embodiment utilizes many of thecomponents described above with reference to FIG. 13A, except that aseparate charging loop 252 is provided between the first redox flowbattery stack 210 and the electrolyte storage tanks 214, 216, and aseparate discharge loop 254 is provided between the second redox flowbattery stacks 210, 212 and the electrolyte storage tanks 214, 216. Forexample, a set of discharge loop pumps 260, 262 pumps electrolyte inletflows 256, 258 from the electrolyte storage tanks 214, 216 into thesecond redox flow battery stack 212, and a set of charging loop pumps268, 270 pumps electrolyte inlet flows 264, 266 into the first redoxflow battery stack 210. This enables the charging and dischargingprocesses to be operated independently of one another. Thus, if demandson the system for discharging electricity require a higher electrolytemass flow rate in the discharge loop 254 than in the charging loop 252,the discharge loop pumps 260, 262 can be operated at a different speedthan the charging loop pumps 268, 270. Similarly, if no dischargingelectricity is required, the charging loop pumps 268, 270 may beoperated to continue charging the system while the discharge loop 254remains idle. Thus, during the off peak evening hours the charging loop252 can be operated to store energy in the reactants while thedischarging loop is operated intermittently as required to meet loaddemands.

The vehicle charging station 250 embodiment illustrated in FIG. 13Bprovides output power 234 to a vehicle charger 272 which is configuredto provide electrical power at the voltage and current density requiredto charge electric powered vehicles 274. This embodiment takes advantageof the load following capacity of the redox flow battery system since itis anticipated that rapid charging of electric vehicles will requirelarge power demands. Since the charging of electric vehicles is unlikelyto be a constant process, and is more likely to occur randomly whenvehicles arrive at the charging station, such periodic requirements forsignificant electrical power would cause unacceptable demands on theelectrical utility grid 202, renewable energy sources 204, and/ordistributed generator sources 205, such as a fuel cell 352. The redoxflow battery system can meet the demand for charging power simply byincreasing the mass flow rate of the electrolytes through the dischargeloop 254. Thus, while the charging loop 252 draws a constant amount ofpower from the utility grid 202, renewable energy sources 204, and/ordistributed generator sources 205, the discharge loop 254 and its secondredox flow battery stack 212 can be operated to meet the periodicdemands for recharging electric vehicles. This embodiment ensures thatvariations in power received from the grid 202 or on-site renewableenergy power sources do not disrupt vehicle charging or damage vehiclestorage batteries. The unique characteristics of the redox flow batterysystem enables DC□DC conversion with high overall system efficiency,further providing an economical vehicle charging system. Also, thecharging station operator can charge the electrolytes during off-peakhours when electricity rates are lower, thereby improving the operator'soverall gross profit margins.

Similar to the embodiment described above with reference to FIG. 13A,the first and second redox flow battery stacks 210, 212 are configuredin design for their respective functions of charging and discharging. Inthe Fe/Cr redox flow battery embodiment illustrated in FIG. 13B, thefirst redox flow battery stack 210 is configured for charging so chargecatalyst loading, charge catalyst activity, temperature, mass transportrate, and separator membrane selectivity increase in succeeding cellsalong the flow path from inlet to outlet. In contrast, the second redoxflow battery stack 212 is configured for discharging so dischargecatalyst loading, discharge catalyst activity, temperature and masstransport rate increase, and separator membrane selectivity decreases insucceeding cells along the flow path from inlet to outlet.

FIG. 13C illustrates an alternative embodiment electric vehicle chargingstation 300. This embodiment utilizes many of the components describedabove with reference to FIGS. 13A and 13B, except that valves 302, 304are used to control the electrolyte reactant flows through the chargingloop 252 and discharge loop 254 so that electrolyte reactants are pumpedthrough one or both of the loops by a single set of electrolyte pumps260, 262. This embodiment may have cost advantages since it requiresfewer pumps.

Similar to the embodiments described above with reference to FIGS. 13Aand 13B, the first and second redox flow battery stacks 210, 212 areconfigured for their respective functions of charging and discharging.In the Fe/Cr redox flow battery embodiment illustrated in FIG. 13C, thefirst redox flow battery stack 210 is configured for charging so chargecatalyst loading, charge catalyst activity, temperature, mass transportrate, and separator membrane selectivity increases in succeeding cellsalong the flow path from inlet to outlet. In contrast, the second redoxflow battery stack 212 is configured for discharging so dischargecatalyst loading, discharge catalyst activity, temperature and masstransport rate increase, and separator membrane selectivity decreases insucceeding cells along the flow path from inlet to outlet.

In a fourth example embodiment illustrated in FIG. 13D, the redox flowbattery energy storage system can be used with a fuel cell to provide afuel cell/redox flow battery power generation system 350 for providingreliable load-following power to a power grid or industrial facility.This embodiment utilizes many of the components described above withreference to FIG. 13A. In this embodiment, electrical power is receivedfrom a fuel cell 352 which generates electricity from the chemicalconversion of a fuel, such as hydrogen, received from a fuel source 356.Fuel cells are very efficient generators of electricity which produceless pollution that most other fuel-based energy generation systems. Asis well-known, fuel cells operate most efficiently and last longer whenoperated at a constant output power level. However, the power demand ona typical utility grid 202 or an industrial facility 359 fluctuatessignificantly throughout the day. Thus, while fuel cells may represent apromising and efficient alternative source of electrical power, theircharacteristics are ill-suited to utility grid application. Thisembodiment fuel cell/redox flow battery system 350 overcomes thislimitation of fuel cells by using dual redox flow battery stacks 210,212 to enable simultaneous charging and discharging operations so thatpower can be received at a fixed power level from the fuel cell 352while meeting the fluctuating demands of the power grid 202 or anindustrial facility 359.

In this embodiment, the chemical fuel, such as hydrogen or natural gas,may be provided from a fuel source 356 via a fuel pipe 354 to the fuelcell 352. For example, the fuel cell/redox flow battery system 350 maybe located at or near a source of natural gas, such as in an oil field,so that natural gas extracted from the ground can be provided to thefuel cell. The fuel cell 352 converts the fuel into electricity andeffluents (e.g., water and carbon dioxide). Electricity output from thefuel cell 352 is provided to the first redox flow battery stack 210where the power is used to charge the electrolytes stored in theelectrolyte storage tanks 214, 216. As described above, electricalenergy stored in the electrolyte species is converted into electricityin the second redox flow battery stack 212. Electricity output 234 fromthe second redox flow battery stack 212 can be provided to an inverter358 which converts the DC current generated by the battery into ACcurrent compatible with the utility grid 202 or industrial facility 359.The inverter 358 may be a solid-state electrical DC□AC inverter or amotor-generator as are well-known in the art. In this embodiment, flowof the electrolyte through the second redox battery stack 212 can becontrolled by adjusting the speed of the pumps 226, 228 so as togenerate electricity to meet the demands of the grid 202. When demandfrom the utility grid 202 or industrial facility 359 exceeds thesteady-state output of the fuel cell 252, stored energy in theelectrolyte is used to meet the additional demand. When demand from theutility grid 202 is less than the steady-state output of the fuel cell252, the excess energy is stored in the electrolyte. Thus, the system350 can follow the peak demands of the utility grid 202 or industrialfacility 359 without having to operate the fuel cell 352 in aninefficient or potentially damaging manner. In a similar but alternativemanner, the system 350 can be used as an on-site distributed generatorto follow the peak demands of a co-located industrial facility load 359.The base load demand of an industrial facility 359 can be satisfied bythe utility grid 202 or an independent stand alone fuel cell system 352.

Similar to the embodiments described above with reference to FIGS.13A-13C, the first and second redox flow battery stacks 210, 212 areconfigured for their respective functions of charging and discharging.In the Fe/Cr redox flow battery embodiment illustrated in FIG. 13D, thefirst redox flow battery stack 210 is configured for charging so chargecatalyst loading, charge catalyst activity, temperature, mass transportrate, and separator membrane selectivity increases in succeeding cellsalong the flow path from inlet to outlet. In contrast, the second redoxflow battery stack 212 is configured for discharging so dischargecatalyst loading, discharge catalyst activity, temperature and masstransport rate increase, and separator membrane selectivity decreases insucceeding cells along the flow path from inlet to outlet.

In a further embodiment illustrated in FIG. 14, a redox flow batterysystem 400 is configured to use gravity to flow reactants through thebattery cells, thereby reducing or eliminating the need for pumps. Thegravity-driven redox flow battery system 400 has fewer components and isless complex than other flow battery systems, thereby reducing itsacquisition costs. Eliminating the pumps also reduces parasitic lossesresulting in a more efficient overall energy storage system. Energy isstored in the chemical species concentrations in the electrolyte storedin the tanks 404, 406. The electrolyte is passed through a redox flowbattery stack 410 which either charges the electrolyte or discharges theelectrolyte depending on the direction of flow and applied power orload. Electrolyte fluid exiting the redox flow battery stack 410 is thencollected in a matching set of tanks 414, 416 positioned below the redoxflow battery stack 410. The illustrated example embodiment includes fourreactant tanks 404, 406, 414, 416, two (404, 414) for the anolytereactant and two (406, 416) for the catholyte reactant. Optional valves418, 420, 424, 422 may be included to enable control or throttling ofreactant flows through the redox flow battery stack 410. The redox flowbattery stack 410 and the four reactant tanks 404, 406, 414, 416 may beintegrated within a support structure 402, such as a cylinder. When thevalves 418, 420, 422, 424 are opened, reactant flows from the top tanks404, 406 through the redox flow battery stack 410 and into the bottomtanks 414, 416 via gravity. In the charge mode, electricity is consumedby the redox flow battery stack 410 at a rate consistent with theelectrolyte flow rate and the state of charge of the electrolyte. Oncethe energy stored in the reactants is replenished or it is otherwisetime to discharge the system, the gravity-driven redox flow batterysystem 400 is rotated 180° so that discharging operations can begin.Thus, the charge/discharge operation of the redox flow battery stack 410depends upon the orientation of the system.

Since the goal of the embodiment illustrated in FIG. 14 is simplicity ofoperation and design, a single redox flow battery stack 410 is used forboth the charging and discharging modes, although separate batterystacks could be used. As described above with reference to FIG. 5, thesingle redox flow battery stack 410 is configured to match catalystloading, catalyst activity, temperature, reactant mass transport rateand separator membrane selectivity to the electrolyte concentrationsexpected in each independent cell along the reactant flow path in thecharging and discharging modes. Specifically, the single redox flowbattery stack 410 is configured so catalyst loading, catalyst activity,temperature and mass transport rate change depending on which half cycle(charge or discharge) requires optimization and separator membraneselectivity increases in succeeding cells from one end of the batterystack to the other. In operation the reactant flows through the redoxflow battery stack 410 in one direction for charging, and in theopposite direction for discharging.

Additionally, since the goal of the embodiment illustrated in FIG. 14 issimplicity of operation and design, the redox flow battery stack 410 andthe tanks 404, 406, 414, 416 may not include thermal management or heatexchangers for controlling the temperature of the reactants.

Operation of the gravity-driven redox flow battery system 400 isillustrated in FIGS. 15A-15C. In the charging mode illustrated in FIG.15A reactant flows from the top tanks 404, 406 through the redox flowbattery stack 410 and into the bottom tanks 414, 416 via gravity whileelectrical power is applied to the battery stack. Flow of the reactantsthrough the redox flow battery stack 410 may be controlled using thevalves 418, 420, 422, 424 to match the amount of charging power beingapplied to the stack. Thus, when no power is available for charging, thevalves 418, 420, 422, 424 may remain closed, and when less than fullcharging power is available the valves 418, 420, 422, 424 may bepartially opened to provide a metered flow through the battery stack410. The redox flow battery stack 410 and the tanks 404, 406, 414, 416are plumbed with flow directing piping and configured so that thereactant flows through the battery stack during charging in thedirection in which the catalyst loading, catalyst activity and masstransport decrease and the separator membrane selectivity increases frominlet to outlet.

As illustrated in FIGS. 15A-15C, the redox flow battery system 400 maybe integrated within a cylindrical support structure 402 that issupported on rollers 430, 432 or an axle (not shown) so that the systemcan be rotated about its long axis to shift from charging to dischargingmodes or discharging to charging modes. For example, in an embodiment,one or more of the rollers 430, 432 may be equipped with a drivemechanism, such as an electric drive motor (not shown), a chain drivenmechanism (which may couple with a motor or bicycle peddles, forexample) or a simple hand crank 434 to, enable rotation of thecylindrical support structure 402. This operation is illustrated in FIG.15B which shows the valves 418, 420, 422, 424 closed and the cylindricalsupport structure 402 being rotated in the clockwise direction byrotation of a hand crank 434 drive mechanism connected to one of therollers 432. The hand crank 434 illustrated in FIG. 15A-15C is forillustration purposes only as a variety of mechanical power sources maybe used as the drive mechanism, such as a chain-drive connected to abicycle, an electric or internal combustion motor, a water wheel, etc.

As illustrated in FIG. 15C, rotating the redox flow battery system 400through 180° places the system in the configuration for dischargingoperations so that charged reactant from tanks 414, 416 flows throughthe redox flow battery stack 410 and into the bottom tanks 404, 406 viagravity, thereby generating electricity from the battery stack 410. Dueto the configuration of the system, the reactants flow through the redoxflow battery stack 410 in a direction opposite that during charging.Flow of the reactants through the redox flow battery stack 410 may becontrolled using the valves 418, 420, 422, 424 to match the amount ofelectrical power that is generated. Thus, when no power is required thevalves 418, 420, 422, 424 may remain closed, and when less than fullcapacity power is required the valves 418, 420, 422, 424 may bepartially opened to provide a metered flow through the battery stack410.

The advantage of eliminating pumps from the flow battery system in theembodiment illustrated in FIGS. 14-15C are several fold. First, theembodiment enables the system to be fully sealed. It is very importantfor the redox flow battery system to be completely sealed as any leakageof air into the electrolyte tanks or pipes will oxidize the reactantthereby reducing performance and potentially generating dangerous gases.Therefore, a very well sealed system is important. Eliminating the needfor pumps ensures a more robust and simplified closed system. Second,eliminating pumps improves overall system efficiency. Pumps are a sourceof parasitic losses which directly reduces the roundtrip efficiency ofthe system. Thus, this embodiment maximizes roundtrip efficiency,especially if the rotation is performed with cheap energy, e.g., amanual crank 434. Third, eliminating the need for pumps reduces the costand maintenance requirements since the acidic nature of the electrolytereactants require special pumps and pump materials. Fourth, the methodused to rotate the structure 402 does not contact the reactants, so lowcost, reliable mechanisms, including human power, can be used to rotatethe system to shift operating modes. Fifth, system operation is quiet asthere are no moving parts when the system is operating.

The control valves 418, 420, 422, 424 are the only moving mechanicalcomponents apart from the rotation mechanism. The system can be operatedflexibly by switching between charge and discharge mode at any time. Forexample, once the system has discharged through one cycle it may beadvantageous to discharge a second time by rotating the system through180° to flow reactants back into the proper tanks for dischargingwithout applying power to the battery stack 410, and then rotating thesystem another 180° to restart the discharge process. Doing so willgenerate more electrical power stored in the reactants, although thepower output will be lower than the first discharge cycle. Likewise thesystem can be charged through a number of cycles in a similar process.Also the system can switch from charge to discharge mode without theneed to rotate the tanks if needed, although the efficiency of thesystem will be reduced.

The simplicity of design and operation of the embodiment described abovewith reference to FIGS. 14 and 15A-15C, as well as the safety of theFe/Cr electrolyte reactants, makes the embodiment system ideal for smallpower storage applications. For example, this embodiment may be ideallysuited for use in remote power applications, such as remote towns andvillages beyond the reach of a utility grid that use solar photovoltaicarrays and/or wind turbine generators for electricity. Adding a redoxflow battery system similar to this embodiment would allow remote townsand villages to be supplied with electrical power at night, for example.Similarly, one or two systems according to this embodiment may be usedin remote electric vehicle charging stations using utility grid power orlocal renewable energy sources to charge the system when no cars need tobe charged, and rotating the storage system to provide electrical powerfor recharging an electric vehicle when required.

It is also possible to size this embodiment system to fit insidestandard sized shipping containers. Because these systems are fullysealed and self contained they can be safely operated inside theshipping container, enabling the systems to be packaged for rapiddeployment. For transportation purposes the electrolyte may betransported as a salt, e.g., a ferric chloride, which may be stored inthe tanks. This can significantly reduce the weight of the system fortransportation. Then once the system is in place, water can be added toreach the required concentrations for operation. In this manner, systemssuch as the embodiment described above with reference to FIGS. 14-15Ccan be built and stored in a condition ready for immediatetransportation, and moved to a location requiring energy storage whenneeded. For example, such deployable energy storage systems may be setup at natural disaster sites, such as a hurricane landfall or earthquakeepicenter, to help provide emergency electrical power until reliableutility services can be restored.

FIGS. 14 and 15A-15C show the battery stack 410 fully integrated withthe tanks 404, 406, 414, 416, and fixed plumbing within the supportstructure 402. However, in another embodiment the tanks 404, 406, 414,416 may be separated from the battery stack 410 so that the tanks may berotated to achieve the desired gravity feed through the battery stack410 which remains stationary. This alternative embodiment may be moreflexible in terms of the ability to easily add more tank/storagecapacity. This alternative embodiment will require flexible piping orinclude fluid couples that accommodate rotation without leaking.

As mentioned above, the various embodiments utilize independent cellswith different configurations along the reactant flow path to increaseoverall electrical performance. FIGS. 16A-16C show micrographs ofexample separator materials that would be appropriate for use in theindependent reaction cells in a three-cell redox flow batteryconfiguration illustrated in FIG. 2. The separator material illustratedin FIG. 16A, which is appropriate for use in a cell adjacent to thestack inlet in discharge mode and to the stack outlet in charging mode,is made of a microporous material with a membrane porosity of less thanabout 0.1 micron. This microporous material exhibits an area specificresistance of about 0.8 ohm-cm2 and has a reactant selectivity of about2000 μg Fe/hr-cm/M. The separator material illustrated in FIG. 16B,which is appropriate for use in a cell half way between the stack inletand stack outlet, is made of a melt blown material with a membraneporosity of about two to about five microns exhibiting an area specificresistance of about 0.5 ohm-cm2 and having a reactant selectivity ofabout 4000 μg Fe/hr-cm/M. The separator material illustrated in FIG.16C, which is appropriate for use in a cell adjacent to the stack outletin discharge mode and to the stack inlet in charging mode, is made of aspunbond material with a membrane porosity of about 15 to about 30microns exhibiting an area specific resistance of about 0.2 ohm-cm2 andhaving a reactant selectivity of 12,000 μg Fe/hr-cm/M.

Further representative stack design parameters and performancecharacteristics for a three-cell configuration are listed in Table 1below. All values are approximate.

TABLE 1 State of Charge (%) 90-62 62-34 34-6 Utilization (%) 31% 45% 82%Electrolyte Concentration [M] [Cr2+] 1.80-1.24 1.24-0.68 0.68-0.12[Cr3+] 0.20-0.76 0.76-1.32 1.32-1.88 [Fe3+] 1.80-1.24 1.24-0.680.68-0.12 [Fe2+] 0.20-0.76 0.76-1.32 1.32-1.88 Electrode Surface AreaLower Medium Higher Electrode Discharge Lower Medium Higher CatalystLoading Electrode Charge Higher Medium Lower Catalyst Loading ElectrodeResidence Higher Medium Lower Time Separator Selectivity 2,000 4,00012,000 (μg Fe/hr-cm/M) Separator Area Specific 0.8 0.5 0.2 Resistance(ohm-cm²) Separator Porosity (μm) <0.1 2-5 15-30 microns microns microns

The various system embodiments may employ a variety of electrolytestorage tank configurations as described below. In a simple embodiment,a single tank may be used to store each electrolyte as illustrated inFIG. 1. This configuration reduces the number of tanks and may enablerapid switching from charge to discharge modes (and vice versa).However, such a system embodiment will suffer from efficiency lossesfrom mixing of charged and discharged electrolytes.

In a second approach, charged and discharged electrolytes can be storedseparately in system embodiments illustrated in FIGS. 1 and 13 by usingseparate tanks for each, resulting in a total of four tanks in thesystem (i.e., one for each of the charged anolyte, discharged anolyte,charged catholyte, and discharged catholyte). The use of four tanks in abattery system is illustrated in FIGS. 14-15C. Additional pumps andvalves may be used within the system to flow the electrolytes to/fromthe appropriate tank depending upon the charge/discharge mode for theembodiments illustrated in FIGS. 1 and 13A-13D.

In a further embodiment illustrated in FIG. 17, the redox flow batterysystem can be configured with electrolyte storage tanks that minimizemixing of the charged and discharged electrolytes. In such a system, theelectrolyte storage tanks 26, 28 and a flow system are fluidicallycoupled to the redox flow battery stack assembly 10 so that electrolytefluid pumped out of each of the electrolyte storage tanks 26, 28 flowsthrough the redox flow battery stack assembly 10, and then back into thesame tank 26, 28 without diluting charged electrolytes. In thisembodiment, each electrolyte tank 26, 28 will store both chargedreactant 504, 514 and discharged reactant 506, 516, with a tankseparator 502, 512 include in each tank which prevents or at leastinhibits the mixing of charged electrolyte 504, 514 with dischargedelectrolyte 506, 516. This embodiment reduces the number of electrolytestorage tanks required in the system while improving system efficiency.

The tank separator 502, 506 inhibits mixing of the charged electrolyte504, 514 that is fed to the redox flow battery stack assembly 10 withthe discharged electrolyte 506, 516 which flows back into theelectrolyte tanks 26, 28. This prevents dilution of the chargedelectrolytes and keeps the charged electrolyte concentrations at aconstant level through out the discharging cycle, thereby maintainingthe battery cell potentials constant. If mixing were to occur theelectrolyte concentrations in the electrolyte tanks 26, 28 would bereduced over time as more and more discharged electrolyte 506, 516 isreturned to the tanks. FIG. 18 illustrates the impact on cell voltageover time if charged and discharged electrolytes allowed to mix, line552, compared to the cell voltage over time if the charged anddischarged electrolytes are kept separate, line 550. By including a tankseparator 502, 512, a single electrolyte tank can be used for each ofthe anolyte and catholyte reactants while ensuring that batterypotential remains constant throughout the discharge cycle. This savesthe cost of an extra set of tanks. Additionally, by maintaining a moreconstant voltage over the course or charging or discharging, theefficiency of any DC-DC, DC-AC, or AC-DC conversion of the electricitygoing into/out of the redox flow battery stack will be higher thandesigns in which charged electrolyte mixes with discharged electrolyte.This is because these types of converters operate more efficiently innarrower voltage ranges. Lastly, redox flow battery stack output powerwill remain more constant than designs where charged electrolyte mixeswith discharged electrolyte.

While FIG. 18 illustrates the impact on battery discharge potential, asimilar impact on system efficiency will occur if charged electrolyte isallowed to mix with discharged electrolyte during a charging cycle.Thus, the tank separator 502 functions to prevent or reduce mixing ofcharged and discharged electrolyte during both charging and dischargingcycles leading to lower system cost, a more constant power output, andhigher DC efficiency.

The tank separator embodiments include two forms of movable tankseparator designs; a tank separator with flow passages which can beopened to enable electrolyte to flow through the separator, and a tankseparator with no flow passages. Operation of these two embodimentconfigurations are illustrated below with reference to FIGS. 19A-19F and20A-20F.

In a first embodiment illustrated in FIG. 19A-19F, the tank separator502 is formed from a buoyant structure or material which can float onthe electrolyte reactant and includes flow passages which when closedinhibit fluids above and below the separator from intermixing, and thatcan be opened to allow fluids above and below the tank separator to mix.The tank separator 502 may be made, for example, from a polypropylene orpolyethylene material which has a lower density than the acidicelectrolyte fluid and that is resistant to corrosion by the asset. Thetank separator 502 includes a valve mechanism, such as louvers 503 (asillustrated in FIG. 19A-19F), closeable openings, an array of valves, orsimilar structures which can be opened to allow fluid to pass throughthe separator structure. Opening such valve mechanisms will allow thetank separator 502 to float to the top of the electrolyte tank 26 whenthe discharge cycle is over. In the example embodiment illustrated inFIGS. 19A-19F, the tank separator 502 includes a number of louvers 503which may be an arrangement of slats that form a seal when rotated intoa closed position and allow fluid to flow between the slats when rotatedinto an opened position. In another example embodiment, the tankseparator 502 may include a slideable panel on the surface which can beslid to expose a hole through the separator structure which allows thefluid to pass through.

FIGS. 19A-19F show a cross section of an electrolyte tank 26illustrating movement of the tank separator 502 through a full dischargeor full charge cycle of a redox flow battery system. FIG. 19A shows theelectrolyte tank 26 with the tank separator 502 floating on the top ofthe electrolyte liquid 504 with its louvers 503 in the fully closedconfiguration. This configuration reflects the start of a charge ordischarge cycle.

During a charge or discharge cycle, initial (either charged ordischarged) electrolyte 504 is drawn from the tank 26 from below thetank separator 502 and passed through the redox flow battery stackassembly 10 while electrolyte exiting the battery 506 is pumped into thetank 26 on top of the tank separator 502. This is illustrated in FIG.19B which shows the configuration of the electrolyte tank 26 and thetank separator 502 part way through a charge or discharge cycle withincoming electrolyte 506 being pumped into the electrolyte tank 26 ontop of the tank separator 502 while the electrolyte 504 being fed to theredox flow battery stack assembly 10 is drawn from below the tankseparator 502 (flow 34). As shown in FIG. 19B the tank separator 502inhibits mixing of the initial (either charged or discharged)electrolytic liquid 504 with the incoming (either discharged or charged)electrolytic liquid 506.

FIG. 19C shows a portion of the charge or discharge cycle with the tankseparator 502 nearing the bottom of the electrolyte tank 26 as willoccur near the end of a charge or discharge cycle. At this point thelouvers 503 in the tank separator 502 remain closed keeping the chargedand discharged electrolytes 504, 506 separated.

FIG. 19D shows the tank separator 502 positioned near the bottom of thetank 26 where it will be at the end of a charge or discharge cycle. Atthis point the louvers 503 may be opened to allow the electrolyte 506above the tank separator 502 to pass through the separator structure.Since the tank 26 is full of the same type of electrolyte 506 (eithercharged or discharged), the valve mechanisms can be opened and the tankseparator 502 moved without causing an electrical performance penalty.FIG. 19D shows an embodiment in which louvers 503 are opened by rotatingthem into an open position, but another embodiment may allow fluidpassage through the separator by sliding a panel to expose holes throughthe tank separator 502, or opening valves to enable fluid to passthrough pipes through the separator structure.

Since the tank separator 502 is buoyant, opening the louvers 503 (orother valve structures) enables the tank separator 502 to begin floatingtowards the top of the tank. This is illustrated in FIG. 19E which showsthe tank separator 502 floating back to the top of the electrolyte tank26 as the electrolyte 506 flows through the open louvers 503. While thetank separator 502 may simply float to the top, a magnetic coupling mayalso be provided to assist the tank separator 502 in moving back up tothe top.

When the tank separator 502 reaches the top of the electrolyte 506 asillustrated in FIG. 19F the next cycle (either charging or discharging)can begin by closing the louvers 503 in the tank separator 502 asillustrated in FIG. 19A before electrolyte 506 from the redox flowbattery stack assembly is pumped back into the electrolyte tank 26.

Closing or opening the valve structures of the tank separator 502 can becontrolled via an external drive which may be coupled magnetically tothe valve mechanism, such as louvers 503. In this manner no wires orother connections are required between an outside controller or powersupply and the separator. In a redox flow battery system the electrolyteflows through a completely closed system to avoid contact with air. Thismakes it difficult to perform maintenance on the valve mechanisms insidethe electrolyte tank 26 for long periods. Therefore, an external controlmechanism using magnetism as a coupling mechanism, for example, may haveadvantages for controlling the tank separator 502 inside the electrolytetank 26.

Alternatively, the valve mechanisms or louvers 503 may be controlled bymechanical mechanisms activated by the position of the tank separator502 within the tank 26. For example, the valve mechanisms, such aslouver 503 may be configured to shut when the structure surfaces, suchas a buoyant lever that latches the louvers closed when it rises abovethe fluid surface, and may be configured to open upon a portion of thestructure contacting the bottom of the tank, such as a latch releasemechanism.

In an alternative embodiment the tank separator 602 may be verticallyoriented and configured to traverse the length of a horizontallydisposed electrolyte tank 600 as illustrated in FIGS. 20A-20F. In thisembodiment the vertical tank separator 602 does not include louvers orvalve structures, and instead is configured to inhibit the fluids oneither side from mixing at all times. FIG. 20A shows the electrolytetank 26 with the vertical tank separator 602 positioned near the leftend of the electrolyte tank 600 separating discharged electrolyte 606from charged electrolyte liquid 604. This reflects the start of a chargecycle. FIG. 20B shows a portion of the charge cycle with freshly chargedelectrolyte 604 being pumped from the redox flow battery stack assembly10 into the electrolyte tank 600 on one side of the vertical tankseparator 602 while discharged electrolyte 606 exits the electrolytetank 600 to flow through the redox flow battery stack assembly 10. Asshown in FIG. 20B the vertical tank separator 602 inhibits the chargedelectrolyte 604 and discharged electrolyte 606 from mixing. FIG. 20Cshows the system at a point part near the end of the charge cycle withthe vertical tank separator 602 nearing the right end of the electrolytetank 600.

To begin discharging the battery, the direction of the electrolytesflowing through the redox flow battery stack assembly 10 are reversed asshown in FIG. 20D. As discharged electrolyte is pumped into theelectrolyte tank 600, the vertical tank separator 602 traverses backalong the length of the electrolyte tank 600 as illustrated in FIG. 20E.Thus, as the redox flow battery system is discharged, for example, thevertical tank separator 602 traverses the electrolyte tank 600 in theother direction.

At any time the flow through the redox flow battery stack assembly 10can be reversed in order to switch from charging to discharging, ordischarging to charging. Thus, as illustrated in FIG. 20F, before thebattery is fully discharged, the flow can be reversed by pumpingdischarged electrolyte 606 from the electrolyte tank 600 through theredox flow battery stack assembly 10 and back into the electrolyte tank600 on the other side of the vertical tank separator 602, such as toreturn to storing energy.

In the embodiment illustrated in FIGS. 20A-20F the vertical tankseparator 602 may be a plastic member that keeps the charged anddischarged fluid apart to prevent dilution. The vertical tank separator602 in this embodiment does not require external control since itsposition within the electrolyte tank 600 is controlled by the directionof the flow through redox flow battery stack assembly 10. Thus, thevertical tank separator 602 can be a relatively simple plastic panelthat is suspended or configure to move freely horizontally through theelectrolyte tank 600.

The seal between the tank separator 502, 602 and the electrolyte tank26, 600 does not need to exceptionally leak proof because a small amountof leakage around the edges will have very little impact on the overallsystem efficiency if the tank volume is sufficiently large. Also someleakage, while undesirable, does not pose any threat to the flow batterysystem other than slightly reducing its overall efficiency.

Since the tank separator moves due to electrolyte being extracted fromthe tank when it is in one state-of-charge and reinjected when it is inthe opposite state-of-charge, the position of the tank separator canfunction as a state-of-charge indicator. By incorporating a passive oractive signaling device, such as a RFID chip or a large piece of metal,the position of the tank separator and hence the system state-of-charge,can be determined by a position sensitive reader of the signal from theRFID chip or induced magnetic field of the metal piece. Multiple RFIDchips or metal pieces can be used to increase signal strength and/orprovide redundancy.

The horizontal or vertical tank embodiments described in FIGS. 17, 19,and 20 can be used in the system designs described above with referenceto FIGS. 13A, 13B, 13C, and 13D to create a backup power capabilitywithin the systems.

As mention above, the electrolytes stored within the tanks 214, 218 inFIG. 13A also provides a back up power capability in the power system.As an example, when the energizing sources for the charging stack (stack1) in FIG. 13A are disconnected or go down, the discharged electrolytefrom the discharging stack 212 can be directed by a 3-way valve down apiping run (not shown) that bypasses the charging stack 210 and bringsthe discharged electrolytes into the backend of the tank, behind a tankseparator (as illustrated in FIGS. 17, 19, and 20). Charged electrolytethat feeds the discharge stack may be extracted from the front end ofthe tanks 214, 218 and therefore, in front of the tank separator.

Other design approaches may be used to keep charged and dischargedelectrolytes separate. In a first alternative approach, a bladder can beprovided inside each tank for each electrolyte. The bladder could besealed to the tank and be sized appropriately to accommodate the fullvolume of charged and discharged electrolytes. Discharged electrolytemay be pumped into the bladder portion of the tank, with the bladderpreventing the discharged electrolyte from mixing with the chargedelectrolyte in the remainder of the tank. The use of an in-tank bladderis similar to the movable partition embodiment described above withreference to FIGS. 19 and 20 with the tradeoff of a sealed part for amoving part.

In a second alternative approach, a series of tanks is used for eachelectrolyte that in aggregate have a larger volume than the volume ofelectrolyte. The tanks for an electrolyte are coupled to the redox flowbattery stack assembly such that the discharged and charged electrolytesare distributed among the tanks during each half cycle of the batterysystem. This “N+1” configuration obviates the need for a movablepartition or sealed part with the tradeoff of additional plumbing,valves, and control complexity.

Other alternative designs may leverage the fact that in the dischargedstate the two electrolytes in the Fe/Cr mixed reactant system haveidentical chemical compositions. Thus, for a system that is designed tooperate over a state-of-charge range that goes to full discharge (i.e.,zero state-of-charge), a three tank system may be used where a firsttank holds charged anolyte, a second tank holds charged catholyte, and athird, larger tank, holds the combined discharged electrolytes. In afurther alternative design, one tank may be sized to hold at minimum thevolumes of both the anolyte and catholyte. In a further approach, theone tank may include two partitions inside which move from the middle ofthe tank to the two ends. In this alternative, charged anolyte is pumpedinto/out of one end of the tank while charged catholyte is pumpedinto/out of the other end of the tank, and discharged anolyte andcatholytes are pumped into/out of the middle of tank. As dischargedelectrolyte fills the inner section, its expanding volume pushes thepartitions towards each end, compensating for the decreasing volume ofthe charge electrolytes. In a further alternative, bladders may be usedinstead of partitions to create the three separate volumes within asingle tank.

All tanks in the redox flow battery system embodiments described above(except for those illustrated in FIGS. 14 and 15) can be freestandinginside a building, freestanding outdoors, placed inside a below-gradevault, or buried. Additionally, the tanks can be designed to fit withinthe volume of standard shipping containers. This not only makes thetanks easy to transport, when suitably sealed the outer skin of thecontainers can serve as secondary containment for the electrolytes.

Containerizing the electrolyte tanks described above may enable them tobe more easily deployed than tanks that are constructed onsite orrequire custom foundations that must be built onsite. Also packaging thestacks, redox flow battery control system, and the power conditioningsystem inside standard shipping containers can create an entire systemconfiguration that is easily shipped by rail and/or tractor trailer anddeployed with relatively minimal onsite work. Thus, containerized redoxflow battery systems can provide turn-key power energy storage systemsthat need only be connected to a utility grid or other source ofelectric power. A system design in which the containers housing theredox flow battery stacks, control system, and power conditioning systemare placed above containers housing the electrolyte storage tanks yieldsan energy storage system that can be readily transported and set-up atthe destination, and that facilitates control over electrolyte flows andfull or partial draining of the stacks when the battery system is idlefor short or extended periods of time.

In a further embodiment, the redox flow battery stack assembly may beconfigured so that the battery can perform charging and dischargingoperations with reactants flowing in a single direction. In oneconfiguration, electrolyte tanks 26, 28 that allow mixing of charge anddischarge electrolytes, such as shown in FIG. 1, may be used to enablerapid switching between charging and discharging modes for short periodsof time by using an electrical switch 44. While compromises in designparameters may be made, such as favoring charging over discharging toenable such operations, such an embodiment can switch very quickly fromcharging to discharging, or from discharging to charging simply byelectrically switching connections (e.g., via switch 44) between thestack and the charging power source 45 or the load 46. By maintainingreactant flows in one direction through the redox flow battery stackassembly, the delay in switching modes associated with reversingreactant flows can be avoided. In an alternative configuration, multipletanks (e.g., described above with reference to FIG. 14) or separatortanks (e.g., described above with reference to FIGS. 17-19E) may be usedin this embodiment, with valves, pumps and piping configured to directcharge or discharged electrolytes (depending upon the mode of operation)through the redox flow battery stack assembly in a single direction.

Alternative embodiments of redox flow battery systems will now bedescribed with reference to FIGS. 21 through 31.

As used herein, the term “cell” or “electrochemical cell” carries itsordinary meaning, and refers, without limitation to a discretearrangement of structural and chemical elements configured to allow anelectrochemical reaction to occur. In some embodiments, two or morecells may be combined in a single structure to form a cell block. A cellblock can include any number of cells in any configuration. In someembodiments, a cell block will include a common unitary housing, whilein other embodiments, a cell block can include a plurality of individualcells, each having its own separate enclosure while being logicallycombined in a block. A plurality of cell blocks can be combined to forma stack assembly. A stack assembly can include a plurality of cellblocks connected to one another in any electrical and hydraulicconfiguration to form a battery pack that can be charged and/ordischarged as a single unit.

FIG. 21 shows a cross-sectional view of an embodiment of a singleelectrochemical cell 1010 of a flow battery. The cell 1010 generallyincludes a positive electrode space 1014 separated from a negativeelectrode space 1016 by a membrane separator 1012. In some embodiments,the positive 1014 and negative 1016 electrode spaces are entirely filledby respective electrically conductive positive and negative porouselectrodes 1018, 1020. In alternative embodiments, the positive and/ornegative electrode spaces additionally include flow channels configuredto increase electrolyte flow through the chamber without substantiallyreducing reaction efficiency. In the illustrated embodiment, themembrane separator 1012 is sandwiched between and supported by a pair ofseparator support plates 1060, 1062. A pair of backing plates 1066, 1068may surround and enclose the positive and negative electrode spaces1014, 1016. Inlet ports 1072 direct positive and negative electrolytesinto the positive and negative electrode spaces 1014, 1016 respectively,and outlet ports 1074 carry electrolytes out of the cell.

In some embodiments, the backing plates 1066, 1068 are electricallyconductive throughout their entire area. For example, such entirelyconductive plates can be made of carbon or any other suitableelectrically conductive material. In alternative embodiments, thebacking plates 1066, 1068 can be configured in a multi-layer orcomposite material construction in which only the surfaces immediatelyadjacent to and in physical contact with porous electrodes 1014, 1016are electrically conductive. In the latter embodiments, the portions ofthe backing plate 1066, 1068 surrounding the conductive surfaces may beelectrically insulating. In further embodiments, the backing plates1066, 1068 may include a multiple layer construction including anelectrically conductive layer made of a substantially electricallyconductive material, and an outer structural layer made of a materialthat is not necessarily electrically conductive but which providesmechanical and structural support. In some embodiments, the size,material, or other properties of the backing plates 1066, 1068 of eachcell 1010 may be configured based on a cascade-stage position of thecell and/or the expected state of charge of electrolyte reactantsflowing through the cell which may depend upon the position of the cellwithin the cascade. Such optimizations are discussed in further detailbelow.

In some embodiments, inlet and outlet ports 1072, 1074 are formed aschannels in the backing plates 1066, 1068. In alternative embodiments,inlet and outlet ports 1072, 1074 are formed as channels in the membranesupport plates 1060, 1062. In still further embodiments, the inlet andoutlet ports 1072, 1074 include separate and independent structures. Theinlet and outlet ports 1072, 1074 are typically constructed with smallcross sectional areas to minimize shunt currents. In some embodiments,the inlet and/or outlet ports 1072, 1074 may include flow directingstructures to cause proper mixing of the electrolytes as they enterand/or exit each electrode space 1014, 1016. In some embodiments, thecross-sectional dimensions, length, tortuosity or other aspects of theinlet 1072 and/or outlet 1074 ports may be configured based on acascade-stage position of the cell and/or the expected state of chargeof electrolyte reactants flowing through the cell. Such optimizationsare discussed in further detail below.

In some embodiments, the porous electrodes 1018, 1020 may be made fromcarbon fiber or graphite felt materials. Such materials are commonlyavailable, and are typically used in thermal insulation, air or othergas filters and other applications. Carbon felt materials generallyinclude an open mesh of carbon fibers. Other porous, electricallyconductive materials can also be used as porous electrodes in a flowbattery. The porous electrode may be configured with properties, such asoverall thickness, fiber size, fiber density, porosity, average poresize, tortuosity, and other properties, based on a cascade-stageposition of the cell and/or the expected state of charge of electrolytereactants flowing through the cell so as to improve or optimize theelectrical efficiency of the cell and/or the entire flow battery system.Such optimizations are discussed in further detail below.

In some embodiments, materials may be bonded or plated onto one or bothporous electrodes 1018, 1020 in order to modify the rate at whichelectrochemical reactions occur at that electrode. For example, suchmaterials may include electro-catalysts selected to increase a rate atwhich reactions occur. Alternatively, such reaction rate modifyingmaterials may be reaction suppressants which are selected to reduce therate at which select reactions occur. The porous electrode may beconfigured with catalyst and/or suppressant materials, thicknesses andother catalyst and/or suppressant parameters (such as loading andactivity) based on a cascade-stage position of the cell and/or theexpected state of charge of electrolyte reactants flowing through thecell so as to improve or optimize the electrical efficiency of the celland/or the entire flow battery system. Such optimizations are discussedin further detail below.

In some embodiments, the porous electrodes 1018, 1020 may be bonded,welded or otherwise physically and electrically joined to theirrespective backing plate 1066, 1068. In alternative embodiments, theporous electrodes may be formed integrally with at least a portion ofthe backing plate 1066, 1068.

The separator membrane 1012 may be made of any material capable ofallowing ions to pass through while limiting the migration of otherparticles from one side to the other. Several separator membranematerials are known in the art. Separator membranes may also be selectedor configured to improve cell characteristics based on a cascade-stageposition of the cell and/or the expected state of charge of electrolytereactants flowing through the cell. Such optimizations are discussed infurther detail below.

Within some embodiments of a single redox flow battery, individual cellsor groups of cells may use different separator materials than othercells in the battery. For example, the membrane separator 1012 may bemade from different materials in different cells so as to exhibitvarying diffusion selectivity and ionic resistance. These and otherproperties of membrane separators can be configured based on an expectedstate of charge within a cascade stage. Optimization of separatormaterials and material properties based on a cascade-stage position of acell and/or the expected state of charge of electrolyte reactantsflowing through the cell is discussed in more detail below.

One source of losses in a redox flow battery is due to mixing or leakageof reactants along the edges of the membrane separator 1012. Such lossesmay be eliminated by sealing the membrane separator material edges. Suchedge sealing may be accomplished by fusing the material by heating it toan elevated temperature while compressing it, such as with an iron orvise, such as described above with reference to FIG. 8. Alternatively,gaskets may be positioned around the periphery of each cell chamber forsealing. In further alternative embodiments, the separator membrane maybe bonded to a frame or backing material as described, for example in USPatent Application Publication No. 2010/0092757, filed on Oct. 9, 2009.In some embodiments, a separator material may be sandwiched between twolayers of frame material and bonded with adhesives or solvents. Anyother method of sealing a separator membrane to edge-leakage can also beused in connection with the systems herein.

Electrolytes used in redox flow batteries are typically substantiallycaustic, i.e., significantly acidic or basic aqueous solutions.Therefore, any flow battery components directly contacting the liquidelectrolyte are preferably made of or coated with materials that willnot significantly dissolve or degrade in contact with the electrolyte.Additionally, it is desirable to prevent the dissolution of additionalmetals into the electrolytes, since such additional metals can act asimpurities, which can significantly negatively affect flow batteryperformance.

FIGS. 22 and 23 illustrate embodiments of a flow battery cell block 1200made up of a plurality of single cells 1010 combined and assembled intoa single cell block structure 1200. The flow battery cell block 1200illustrated in FIGS. 22 and 23 efficiently utilizes thin layers ofvarious materials to form a block of many cells in a relatively smallpackage. The illustrated cell block 1200 may be configured such thatelectrolytes flow in parallel through all cells in the block beforeexiting the block.

FIG. 22 illustrates an exploded view of one embodiment of a 16-cellblock 1200. Components of the cell block will be described fromleft-to-right as shown. For simplicity of description, the first cell1010 a of the block 1200 will now be described assuming the left sideelectrode of the first cell 1010 a is a positive electrode 1014. As willbe clear from the figures and the following description, the cell block1200 could just as easily be arranged such that the left side electrodeof the first cell 1010 a is negative. The cell block 1200 generallyincludes a plurality of cell elements sandwiched between outer commonelements 1204.

From left-to-right the common elements of the example cell block 1200include an outer structural support plate 1206, four electrolyteinlet/outlet ports 1208, a fitting plate 1210, and an electric terminalplate 1212. A first electrochemical cell 1010 a includes a firstconductive bipolar plate 1214, a first positive half-cell 1201 includinga first positive-side gasket 1216, a first positive porous electrode1218, and a first catholyte flow channel layer 1220. A first separatorassembly 1222 includes a separator membrane 112 a sandwiched betweenfirst 1124 a and second 1224 b window frames. To the right of the firstseparator assembly 1222 may be a first negative half-cell 1203 includinga first anolyte flow channel layer 1230, a first negative porouselectrode 1232 and a first negative-side gasket 1234. A second bipolarplate 1236 may enclose the negative half-cell and be shared with thesecond cell 1010 b which lies to the right of the second bipolar plate1236. The second and subsequent cells may include all of the sameelements as the first cell 1010 a, with bipolar plates being shared byadjacent cells. The outer common elements 1204 may also be repeated atthe right side of the stack. These include a second terminal plate 1212,a second fitting plate 1210, a second group of inlet/outlet ports 1208and a second outer structural support plate 1206. All elements of thecell block 1200 may include fluid pass-through holes 1246 through whichelectrolyte will flow.

This simple layered design allows additional cells to be added withminimal effort. Thus, while the illustrations of FIGS. 22 and 23 showcell blocks 1200 with 16 cells 1010, cell blocks 1200 of this designcould also be assembled with any number of cells.

The structural support plates 1206 generally provide structural rigidityto the cell block 1200, and may therefore be made from a substantiallyrigid material. Since the support plates 1206 are not in direct contactwith the caustic electrolytes, they can be made from low-cost,high-strength materials such a steel or aluminum. In alternativeembodiments, the support plates can be made of titanium or other morecostly metals. In further alternative embodiments, the support plates1206 can be made of high-strength polymers, such as polycarbonate orsome forms of HDPE. The support plates 1206 are preferably of asufficient thickness and strength that they provide the desired degreeof rigidity at the center of the plate to counteract internal pressures,but sufficiently thin to fit within desired physical dimension andweight requirements. In some embodiments, the support plate 1206 may becoated or painted with a non-reactive and non-conductive coating.

The fitting plate 1210 may provide electrical insulation (if needed) anda hydraulic seal between the support plate 1206 and the conductiveterminal plate 1212. The fitting plate may also provide a surfaceagainst which the fluid inlet and outlet ports 1208 will seal once thecell block 1200 is assembled. As such, the fitting plate 1210 ispreferably made of an electrically insulating material that is alsoimpervious to the caustic electrolytes. Thus, in some embodiments, thefitting plate may be made of polypropylene, polyethylene, high densitypolyethylene, polyvinyl chloride, or another suitable material. Thefitting plate is typically between about 0.25 inches and about 1.0 inchthick. In some embodiments, the thickness of the fitting plate 1210 maybe selected based on characteristics of a seal with the inlet/outletports. In some embodiments, the inlet/outlet ports may solvent welded tothe fitting plate 1210.

In some embodiments, the fitting plates 1210 and/or the support plates1206 may include feet 1240 projecting beyond the outer boundaries ofother cell block elements, thereby providing a support stand for thecell block 1200. In alternative embodiments, the illustrated feet 1240may be replaced by larger and/or differently shaped projectionsconfigured for attaching the cell block 1200 to other support structuresas needed.

The terminal plate 1212 may include terminal tabs 1242 providing anelectrical connection point for electrically joining the cell block 1200to a charge/discharge circuit, other cell blocks, measurement/monitoringequipment, or any other electric circuit. Electrical connections may bemade with simple clamps or alligator clips. Alternatively, morepermanent electrical connections may be made by bolting or otherwisejoining electrical contacts through holes in the tabs 1242. The terminalplate 1212 may physically contact an outer bipolar plate 1214 to captureand conduct electric current from the cells through the outer bipolarplate 1214. Therefore, the terminal plate 1212 is preferably made of ahighly electrically conductive material. In the illustrated embodiment,the terminal plate 1212 is insulated from the caustic electrolytes bythe outer bipolar plate 1214. Thus, in the illustrated embodiment, theterminal plate 1212 may be made of copper, aluminum, or other relativelyinexpensive, electrically conductive materials. In some embodiments, theterminal plate 1212 may be omitted by providing a conductive surface onthe fitting plate 1210.

In some embodiments, a rubber, silicone or other non-reactive,compressible plastic ring gasket 1244 is positioned withinelectrolyte-pass-through holes 1246 in the terminal plate 1212. The ringgasket 1244 may be sized such that its outer diameter fits within theinner diameter of a pass-through hole 1246 in the terminal plate 1212. Acorresponding pass-through hole 1246 in the bipolar plate 1214 may besized such that its internal diameter is substantially the same as aninternal diameter of the ring gasket 1244. Thus, in an assembled state,the ring gasket may be compressed between an inlet/outlet port 1208 andthe outer surface of the outer bipolar plate 1214. The ring gasket sealsthe electrolyte passageway, preventing electrolyte from contacting theterminal plate 1212.

In some embodiments, a washer 1245 may be provided which fits inside theinner diameter of a pass-through hole 1246 in the bipolar plate 1214 andseals against a portion of the ring gasket 1244 to protect an internaledge surface of the bipolar plate from the caustic electrolytes. In someembodiments, the washer 1245, which may be made of any suitable plasticmaterial, may be bonded to or integrally formed with the flow channellayer 1220 and/or 1230. In alternative embodiments, an edge surface ofthe bipolar plate may be treated and/or sealed to resist electrolyteabsorption or degradation.

In some embodiments, the first bipolar plate 1214 may be a solid carbonplate. The purpose of the bipolar plate is to collect electric currentfrom the porous electrode and electrolyte which directly contacts thebipolar plate 1214 in the assembled cell block 1200. Thus, the bipolarplate 1214 may be made of a highly electrically conductive materialimpervious to the caustic environment of the electrolytes. A bipolarplate thickness is preferably selected by balancing a desiredconductivity with the mechanical strength of the plate. A thinner plateis better for electrical conductivity, but some degree of mechanicalstrength may be needed. In some embodiments bipolar plates may have athickness of 0.5 mm or less, depending on the mechanical strengthneeded. Plates with a greater thickness, e.g., up to 1 mm or more, mayalso be used.

In alternative embodiments, the terminal plate 1212 may be integrallyformed with the outer bipolar plate 1214, such as by providingelectrical connection points directly on the bipolar plate 1214.Alternatively, electrical connection tabs of copper, aluminum or anothersuitable metal may be welded, soldered, mechanically joined or otherwiseattached to portions of the outer bipolar plate 1214.

In the cell block of FIG. 22, a first positive electrode chamber may beformed by the interaction of a combination of elements. Thepositive-side gasket 1216 may be sized and shaped to substantiallyexactly surround the catholyte flow channel layer 1220, thereby sealingthe periphery of the flow channel layer 1220. The positive-side gasket1216 seals against the outer bipolar plate 1214 and the first separatorwindow frame 1222. Thus, the positive electrode space is defined as thespace within the internal cutout of the catholyte flow channel layer andbetween the right surface of the first bipolar plate 1214 and the leftsurface of the separator membrane 1012 a.

The gasket 1216 may have a thickness sized to be substantially equal toa thickness of the flow channel layer 1220 in an assembled state. Insome embodiments, the gasket 1216 may be sized to be slightly thickerthan the flow channel layer 1220 such that the thickness of the gasket1216 can be reduced under the clamping pressure. For example, in oneembodiment, a flow channel layer has a thickness of about 0.118 inch anda gasket 1216 has a thickness of about 0.125 inch. The gasket 1216 maybe made of any suitable material capable of being compressed and sealingagainst the bipolar plate 1214 and the separator window frame 1222. Forexample, the gasket 1216 may be made of rubber, silicone, ethylenepropylene diene monomer (EPDM) rubber, or any other suitable material.

The porous electrodes 1218, 1232 are preferably sized to fit exactlywithin their respective electrode space. In some embodiments, the porouselectrodes 1218, 1232 may be sized to be slightly compressed when thecell block is assembled. In alternative embodiments, the porouselectrodes 1218, 1232 may be sized to have substantially zero clearancewithin the electrode space. For example, in one embodiment, thethickness of the electrode space may be equal to the thickness of theflow channel layer 1220. In some embodiments, this electrode spacethickness is about 3 mm (0.118 in.) and the thickness of the porouselectrode is nominally about 5 mm. In alternative embodiments, thepositive and/or negative electrode spaces may be sized and/or configuredto include flow channels configured to decrease an internal pressurewithin the electrode space without substantially reducing reactionefficiency.

The porous electrodes and corresponding electrode spaces may beconfigured with a variety of combinations of aspect ratio (i.e.,length/width), volume and total surface area. Electric power generatedby a flow battery reaction cell is proportional to the active area ofthe electrodes and separator membrane. In some embodiments, theelectrodes may be sized with an aspect ratio of about 1:1 (i.e.,square), 1:2, 2:1, or any other ratio as desired. The separator membranewill typically have an aspect ratio and dimensions corresponding to theelectrodes. In some embodiments, the electrodes and/or separatormembranes may have a shape other than rectangular.

The flow channel layer 1220 includes an inlet 1250 in fluidcommunication with a fluid pass-through hole 1246. Flow channels 1254join the pass-through holes 1246 of the flow channel layer 1220 with theelectrode space 1256 in the interior of the flow channel layer 1220. Insome embodiments, inlets and/or outlets in the flow channel layer 1220may include pressure-reducing deltas shaped like a river delta, in orderto facilitate smooth flow of electrolyte into and/or out of theelectrode space. In some embodiments, the flow channel 1254 includes acutout extending entirely through the flow channel layer 1220. Inalternative embodiments, the flow channel 1254 includes a trench in onlyone side of the flow channel layer 1220. In still further embodiments,the flow channel 1254 includes a trench on both sides of the flowchannel layer with a strip of material in between.

The flow channel layer 1220 may be made of a plastic material such aspolyethylene, polypropylene, polyvinyl chloride or any other materialcapable of withstanding the caustic environment of the electrolytes.

The separator window 1222 includes a top layer 1224 a a membraneseparator 1012 a, and a bottom layer 1224 b. In the illustratedembodiment, the top layer 1224 a and the bottom layer 1224 b may besubstantially identical to one another. In alternative embodiments, thetop layer 1224 a and bottom layer 1224 b may be minor images of oneanother. In some embodiments, the separator membrane 1012 a may besecured between the top 1224 a and bottom 1224 b window layers bycompression alone. In alternative embodiments, the separator membrane1012 a may be secured between the top 1224 a and bottom 1224 b windowlayers by an adhesive, a solvent, heat bonding, welding, or any othermethod or procedure.

As can be seen in FIG. 22, the negative flow channel layer 1230,negative porous electrode 1232, negative-side gasket 1234 and the secondbipolar plate 1236 may be all identical to the same elements on thepositive side of the separator window 1222. The negative flow channellayer may be rotated 180 degrees about a vertical axis relative to thepositive flow channel layer 1220. This re-use of elements within thecell block 1200 advantageously reduces the number of different parts,thereby reducing manufacturing costs.

As will be clear to those skilled in the art, the second bipolar plate1236 collects a negative electric charge on its left surface and apositive electric charge on its right surface (in this example). In thisway, all cells in a flow battery cell block 1200 of this design may bejoined in electrical series with one another. Any number of additionalcells may be added by repeating the positive half-cell 1201, separatorwindow 1222, and negative half-cell 1203 elements described above.

In some embodiments, the layered components of the cell block 1200 maybe clamped together with bolts 1260 extending through bolt holes in eachelement. The size, number and arrangement of bolt holes may be varied asneeded for a particular set of design requirements. In the illustratedembodiment, a cell block includes 14 bolt holes around the periphery ofthe each element. This provides adequate clamping pressure to ensuretight seals at all fluid junctions within the cell block.

For the purpose of the present example, the top-left inlet/outlet port1208 in the first (i.e., left) structural support plate 1206 will bedescribed as a catholyte inlet port. Continuing with this example,operation of a cell block embodiment illustrated in FIG. 22 will now bedescribed.

As catholyte is pumped into the inlet port 1208, the catholyte may passthrough the ring gasket 1244 without contacting the first terminal plate1212. Catholyte then may pass through the corresponding pass-throughholes 1246 in the first bipolar plate 1214 and the first positive flowchannel layer 1220. Once sufficient downstream pressure builds up,catholyte will flow through the inlet channel of the flow channel layer1220 and into the positive electrolyte space occupied by the positiveporous electrode 1218. After flowing through the electrode 1218, thecatholyte may exit the positive electrode space through the outletchannel in the flow channel layer 1220 and then through the lower-rightpass-through hole 1246. The catholyte may then bypass the negativeelectrode space by passing through the lower-right pass-through holes1246 in the negative-side gasket 1216 and the second bipolar plate 1214.Catholyte may then enter the positive electrode space of the second cellvia the bottom-right pass-through hole.

FIG. 23 illustrates an assembled cell block of the embodiment of FIG.22. For description purposes, the eight inlet/outlet ports 1208 in thecell block 1200 are labeled A-H clockwise beginning with the upper-leftinlet port in the left support plate 1206. In the illustratedembodiments, diagonally-opposite inlet/outlet ports form inlet/outletpairs for the same electrolyte. For example, if the first top-left portA is selected as the catholyte inlet port, the bottom-right port on theopposite end of the cell block H will be the catholyte outlet port. Inthe same example, electrolyte can be easily pumped in the oppositedirection, thereby making port H the inlet, and port A the outlet.

Under normal operation, the ports exactly opposite the active ports willbe closed, such as by valves or caps. For example, if port A is used asthe catholyte inlet port, port E will be closed, thereby forcingelectrolyte through the cells and out of the cell block through port H.If it is desirable to bypass the entire cell block (using the sameexample), port E can be opened while optionally closing port H.Electrolyte will thereby be directed through the top-left fluid bypassholes in every cell block element. With port E open, this bypassoperation is likely to occur in substantially the same way even withoutclosing port H, since an incompressible electrolyte will tend to followa path of least resistance to flow. Thus, depending on the desiredconfiguration and mode of operation, any of the eight ports may be aninlet or an outlet for either anolyte or catholyte.

The inlet/outlet ports 1208 may be hydraulically connected to tanks,pumps and/or other cells in any configuration as needed to form acomplete flow battery stack assembly. For example, by joining aplurality of cell blocks 1200 in hydraulic series, such that outletports of a first cell block are joined to inlet ports of a second cellblock (via inter-stage components, in some embodiments), a multi-stagecascade can be assembled. In such a system, each cell block makes up acascade stage with a plurality of cells. In some embodiments, componentsof a single block/stage may be configured according to the expectedstate-of-charge of electrolytes at that point in the cascade asdiscussed herein.

FIGS. 7B, 7C, 24 and 25 illustrate embodiments of flow battery stackassemblies including multiple electrochemical cells configured in a“cascade” or “stepped” arrangements. In a cascade arrangement,electrolyte is directed to flow in series through a plurality of stepsor stages. At each stage along the series, electrolyte reacts inside oneor more electrochemical cells before proceeding into the next stage. Astage may include a single cell, a single cell block made up of multiplecells, or a plurality of cell blocks.

Embodiments of redox flow battery systems described hereinadvantageously include cells or cell blocks in engineered cascadearrangements as shown, for example, in FIG. 24. A cascade redox flowbattery includes a plurality of electrochemical cells arranged instages, such that electrolyte flows from a storage tank 1034, 1036, tothe cell or cells of a first stage 1052, then from the cell or cells ofthe first stage 1052 to the cell or cells of a second stage 1054.Electrolyte then flows from the second stage 1054 to the cell or cellsof a third stage 1056 and from the third stage 1056 to the cell or cellsof a fourth stage 1058, etc. During a discharge cycle, as electrolytestravel through and react in the cell or cells at each cascade stage, thechemical compositions of the electrolytes change in a way that decreasesthe overall state-of-charge of the battery, thereby creating astate-of-charge gradient between electrolyte in a first stage 1052adjacent an inlet end of the cascade flow path, and electrolyte in afinal stage 1058 adjacent an outlet of the cascade flow path.

For example, the system of FIG. 24 can be described as a cascade flowbattery in a discharge cycle with electrolytes flowing from left toright. As shown, the electrolytes entering the first stage 1052 have astate-of-charge of about 90%. The electrolytes will react in theelectrochemical cell or cells of the first stage 1052 such that theelectrolyte concentrations change from a first (higher SOC) state to asecond (lower SOC) state. After reacting in the cell or cells of thefirst stage 1052, the electrolytes will have a decreased state-of-charge(i.e., an overall decreased stored electrical energy), such thatelectrolytes entering the second stage will have an overallstate-of-charge of about 70%. The electrolytes will againelectrochemically react in the cell or cells of the second stage 1054,thereby reducing the electrolyte concentrations from a second state ofabout 70% state-of-charge to a third state of about 50% state-of-charge.The electrolytes will again electrochemically react in the cell or cellsof the third stage 1056, thereby further reducing the electrolyteconcentrations to a fourth state of about 30% state-of-charge. Theelectrolytes will react in the fourth stage 1058, further reducing thestate-of-charge to a fifth state of about 10% state-of-charge.

In an engineered cascade arrangement, the configurations of the cells ateach stage may be configured for the expected state-of-charge conditionof the electrolytes at that stage or in those cells. Therefore, physicaland chemical characteristics of the cell or cells in the first stage1052 may be configured so as to provide conditions for an efficientelectrochemical reaction with the expected state of charge of theelectrolytes ranging from the first state (e.g., about 90% SOC) to thesecond state (e.g., about 70% SOC), and the cell or cells of the secondstage 1054 may be configured so as to provide conditions for anefficient electrochemical reaction from the second state (e.g., about70% SOC) to the third state (e.g., about 50% SOC), and so on.

The number of cascade stages and the number of cells per stage areadditional design variables which may be any desired value. Cascaderedox flow batteries according to the embodiments herein may include anynumber of cascade stages, and any number of electrochemical cells ineach stage. In some embodiments, engineered cascade redox flow batterieswill have between two and ten stages, with each stage having one toabout 20 cells.

In the embodiment of FIG. 24, the flow battery is neither 100% chargednor 100% discharged. In some embodiments, this mode of operation may bepreferred in order to minimize various adverse reactions which occur insome flow batteries at very high and/or very low states-of-charge.However, in other embodiments, electrolyte entering a first stage 1052may have a state-of-charge as high as approximately 100%, and theelectrolyte exiting the last stage (e.g., the fourth stage 58 in theembodiment of FIG. 24) may have an overall state-of-charge ofapproximately 0%.

This engineered cascade flow battery arrangement provides severaladvantages, including limiting shunt currents and improving overallreactant utilization. The engineered cascade flow battery also improvesreactant utilization compared to that of a non-cascade redox flowbattery. Improving reactant utilization helps to improve the roundtripDC efficiency of the redox flow battery and reduces or eliminates theneed to re-circulate electrolytes multiple times through a stackassembly. Recirculation may be disadvantageous because it may involvemore pumping power per kW of stored energy capacity, which increasesparasitic losses. Engineered cascade redox flow battery systems alsohave greater overall electrical storage performance than is possiblewith a cascade arrangement in which cells at all stages are notconfigured based on the expected state-of-charge gradient.

Several physical, structural, chemical and other characteristics andproperties of the components in the electrochemical cells may be variedwhen configuring cells based on the cell's position in the cascade orthe expected condition (e.g., SOC) of reactants in the cell, which maydepend upon the cell's position in the cascade as well as expected ordesign operating conditions of the cascade flow battery system. Forexample, any of the following properties may be varied in configuringcells based on their position in the cascade or expected reactantconditions: catalyst loadings; reaction suppressant loadings; catalystmaterials; reaction suppressant materials; catalyst activities; reactionsuppressant activities; electrode thicknesses; electrode material types;electrode tortuosities; chamber volumes; flow channel dimensions; flowchannel shapes; membrane separator porosities; average pore size ofmembrane separators; separator thicknesses; separator tortuosities; andseparator selectivities. These and other parameters may be adjusted toaccommodate the variations in reactant concentration (andstate-of-charge) expected in each cell according to its position in theflow path of a cascade flow battery. These parameters may be variedindividually or collectively (i.e., two or more of the parameters may beadjusted to the match the cell's expected reactant conditions). Otherfactors that may be considered in selecting and optimizing cellparameters include optimizing coulombic and voltage efficiencies foroperation over a range of current densities.

Blocks of electrochemical cells may be arranged in cascades in anysuitable physical, hydraulic and/or electrical arrangement meeting adesired set of design parameters. FIGS. 7B, 7C and 25 illustrate someembodiments of cascade flow battery designs. However, these are onlyexamples, and many other suitable arrangements utilizing the featuresand advantages described herein are also possible.

FIG. 7B illustrates one embodiment of a cascade redox flow battery stackassembly 10 made up of a plurality of multi-cell layers 48 in a unitaryenclosure. In some embodiments, each layer 48 a, 48 b, 48 c may includea plurality of electrochemical cells in a cascade arrangement. In theembodiment illustrated in FIG. 7B, the collection of “cell 1” cells 52form a first cascade stage, the collection of “cell 2” cells 54 form asecond cascade stage, and the collection of “cell 3” cells 56 form athird cascade stage. A plurality of these multi-cell layers may bestacked together and electrically connected to one another in order toachieve the desired output voltage. For example, in some embodiments,all of the cells may be electrically connected in series. In alternativeembodiments, the cells of a single layer may be electrically connectedto one another in parallel, while layers may be connected to one anotherin series. For example, the “cell 1” 52 a of the first layer 48 a may beelectrically connected in series with the “cell 1” 52 b of the secondlayer 48 b, and the “cell 1” 52 c of the third layer 48 c, while “cell2” 54 a of the first layer 48 a may be electrically connected inparallel to one or more of the “cell 1” cells 52. In an alternativeembodiment, all cells in the FIG. 7B stack assembly may be connected inelectrical series to achieve a higher voltage. Any other electricalconnection arrangement, including various combinations of parallel andseries connections, may also be used depending on the particularrequirements at hand.

In engineered cascade embodiments, the design parameters of each one ofthe first cells 52 a, 52 b, 52 c may be configured according to theexpected state-of-charge of the electrolytes in those cells, and thusmay be different from the design parameters of each second cell 54 a, 54b, 54 c and each third cell 54 a, 54 b, 54 c within the cell layers 48a, 48 b, 48 c.

FIG. 7C illustrates an alternative embodiment in which cells areconstructed independently rather than constructed as multiple cells in asingle unibody frame. Independently constructed cells may then bephysically, hydraulically and electrically connected to one another inany arrangement providing the desired energy storage and deliveryproperties. For example, a plurality of cells can be stacked verticallytogether to form independent stacked-stages, as shown in FIG. 7C. Insome embodiments, the stacked stages may be combined into a unitaryenclosure to form a unitary stack assembly. In alternative embodiments,each stacked stage may have its own enclosure separate from otherstacked stages.

FIG. 25 illustrates an embodiment of a cascade flow battery stackassembly including a plurality of cell blocks 1208, each including itsown enclosure and inlet/outlet ports. The cell blocks of FIG. 25 may bethose of the embodiments illustrated in FIGS. 22 and 23, for example. Inthe embodiment illustrated in FIG. 25, the cell blocks 1200 may beconnected to one another in hydraulic series such that each block is aseparate cascade stage. Thus, the system illustrated in FIG. 25 includesa six-stage cascade, where each stage includes a block of many cells. Insome embodiments, the cells within each cell block may be configuredwith characteristics configured for an expected state-of-chargecondition within the cascade.

FIG. 6 illustrates the performance advantages achieved by configuringelectrochemical cell components based on the position of the cell withina cascade (i.e., the cell's cascade-stage position) or on the expectedstate of the reactants flowing through the cell (which, again, willtypically depend upon its position in the cascade). The polarizationcurve 122 illustrates the output voltage as a function of output currentof a conventional redox flow battery that does not include engineeredcascade cells. This poor performance curve 122 falls well below theideal performance curve 120 which may be approached by the embodimentsof redox flow battery designs implementing the engineered cascadeconfigurations of the various embodiments.

FIG. 26 summarizes some of the design configuration parameters that maybe controlled and the manner in which the parameters are varied alongthe reactant flow path in order to improve electrical performance ofeach independent cell in the redox flow battery based on its positionand/or the expected state of reactants flowing through the cell. Asillustrated in design trend line 1112, for some designparameters—illustrated as Group A parameters—the performance of cellsmay be improved in a discharge mode by may be configured byquantitatively decreasing the parameter from a higher value in the firststage (i.e., closest to the stack assembly inlet) to a lower value atthe nth stage (i.e., closest to the stack assembly outlet). For the sameGroup A parameters performance of cells in a charging mode can beimproved by quantitatively increasing the parameter from a lower valuein the first stage (i.e., closest to the stack assembly inlet) to ahigher value at the nth stage (i.e., closest to the stack assemblyoutlet). In some embodiments, stages between the first and nth stagewill have properties generally between the properties of the first stageand properties of the nth stage. In some embodiments, the design trendline is linear as shown in FIG. 26, while in other embodiments, designtrend lines may follow another curve, but will typically exhibit thestart-to-finish differences discussed above.

As illustrated in design trend line 1116, for other designparameters—illustrated as Group B parameters—the performance of cellsmay be improved in a discharge mode by quantitatively increasing theparameter from a lower value in the first stage (i.e., closest to thestack assembly inlet) to a higher value at the nth stage (i.e., closestto the stack assembly outlet). For the same Group B parameters theperformance of cells may be improved in a charge mode by quantitativelydecreasing the parameter from a higher value in the first stage (i.e.,closest to the stack assembly inlet) to a lower value at the nth stage(i.e., closest to the stack assembly outlet). In some embodiments,stages between the first and nth stage will have properties generallybetween the properties of the first stage and properties of the nthstage. Such intermediate stages will generally have Group B propertiesfalling along the trend line 1116.

In some flow battery systems, the reaction rate of the anolyte chargereaction may be more limited than other reactions. In such embodiments,it may be advantageous to configure each cell design parameters based onthe design curve 1112 shown in FIG. 26. As illustrated, the Group Adesign parameters that may be varied to improve battery cell performancebased on the position in the cascade (and thus state of reactantsflowing through the cell) according to design trend line 1112 include:separator membrane selectivity; separator area specific resistance;separator thickness; electrode felt porosity; electrode residence time;charge catalyst loading; charge catalyst activity; reaction suppressantloading; reaction suppressant activity; temperature (when configuringcells for charging); chamber volume (when configuring cells forcharging); and reactant mass transport (when configuring cells forcharging).

In some embodiments of an engineered cascade flow battery stackassembly, cells may be configured with design parameters selected forthe average electrolyte concentration (i.e., state-of-charge condition)expected within each cascade stage, which may provide a stair stepapproximation of the design trend lines illustrated in FIG. 26. Byincreasing the number of independent cascade stages along the reactantflow path, the cell design parameters can better match the design trendlines to improve the flow battery's energy efficiency.

The Group B design parameters that may be varied to configure batterycell designs according to design trend line 1116 include: ionicconductivity; separator average pore size configuring cells forseparator diffusion rate configuring cells for electrode surface areaconfiguring cells for discharge catalyst/suppressant loading; dischargecatalyst/suppressant activity; temperature (when optimizingdischarging); chamber volume (when optimizing discharging); and masstransport rate (when optimizing discharging).

In some embodiments, only one or two parameters may be adjusted inconfiguring cells for each cascade stage position. In alternativeembodiments, as many as all of the parameters described herein may beadjusted in configuring cells for each cascade stage position. In someembodiments, configuration design choices may be made on a cost-savingsbasis, by selecting higher-cost elements for stages with particularlyhigh and/or low state-of-charge conditions, while using less-costlyelements for stages with state-of-charge conditions closer to 50%. Thus,cells may be configured based on their location in the cascade orexpected reactant state to achieve performance that is less than somepossible or theoretical optimum in order to meet a variety of design,cost and performance objectives.

Embodiments of further representative engineered cascade stack assemblydesign parameters and performance characteristics for a three-stageengineered cascade configuration are listed in Table 2 below. Ranges ofthe state-of-charge of reactants at each stage are provided forreference, although alternative embodiments may utilize differentstate-of-charge ranges.

TABLE 2 Cell Configuration Design Variables vs. Expected State-of-ChargeExpected State of Charge (%) 90%-62% 62%-34% 34%-6% Electrode SurfaceArea Lower Medium Higher Electrode Felt Porosity Higher Medium HigherElectrode Discharge Lower Medium Higher Catalyst Loading ElectrodeCharge Higher Medium Lower Catalyst Loading Electrode Residence TimeLonger Medium Shorter Separator Selectivity More Medium Less SelectiveSelective Separator Area Specific Higher Medium Lower ResistanceSeparator Diffusion Rate Lower Medium Higher Separator Average Pore sizeSmaller Medium Larger Separator Thickness Thicker Medium Thinner

Materials used for porous electrodes in a redox flow battery systemtypically include porous carbon felt materials. Carbon felts aregenerally available for use in a variety of applications, includingthermal insulation. Carbon felts made for insulation are available atrelatively low costs, but are not necessarily sufficiently uniform intheir porosity or average pore size. Carbon felts with controlleddesigned characteristics may be obtained by custom manufacturingprocesses, but such custom materials will typically be more costly. Forexample, it may be desirable to use a carbon felt with a relatively highporosity (i.e., a larger ratio of open volume to solid volume) in orderto increase a volumetric flow rate of electrolyte through the felt. Itmay be more desirable to control reactions at relatively high and lowstates-of-charge than at state-of-charge conditions closer to 50%.Therefore, in some embodiments, higher cost, optimally designed carbonfelts may be used in cascade stages with high and/or low state-of-chargeconditions, while using less costly felts in stages with expectedstate-of-charge conditions closer to 50%.

Chamber volume refers to the interior volume of the cell housing,including the space occupied by the combination of any bonding layerbetween the bipolar plate and negative electrode, the negativeelectrode, the membrane separator, the positive electrode, and anybonding layer between the positive electrode and the bipolar plate. Insome embodiments, configuring the volume of the cell chamber based onposition or expected reactant state may entail increasing the volume ofthe positive or negative electrode side to offset the volume occupied byany gases formed during operation. In an Fe/Cr flow battery system,hydrogen is typically generated at the negative electrode during adischarge reaction, particularly at high state-of-charge. Thus, in someembodiments, a cascade flow battery system utilizing an Fe/Cr chemistrycan be configured with a larger negative electrode chamber volume incells within stages with higher expected state-of-charge conditionsrelative to other stages in the cascade.

In some embodiments, electrochemical cells may be configured for theircascade-stage position (or expected reactant state) by varying factorsaffecting the reactant mass transport rate through the cell. Thereactant mass transport rate is defined as the mass flow rate ofelectrochemical reactants passing through and contacting a porouselectrode within a cell. The effective reactant mass transport ratethrough a cell can be varied by increasing or decreasing the physicalvolume of the cell chamber. For example, decreasing the cell volume willeffectively increase the volumetric flow rate of electrolyte within thecell, which will tend to induce more turbulent flow through the porouselectrode. Increased turbulence will tend to increase the rate thatreactants come into contact with electrode surface, thereby increasingthe mass transport rate.

Alternatively, the mass transport rate of a cascade stack assembly canbe varied by providing stages with cells having a more restricted flowarea closer to one end of the flow path and a more open and lessconstricted flow area at the other end. In such embodiments, thereactant mass transport rate increases in each stage along the reactantflow path when operated in the charge mode, and decreases in each stagealong the reactant flow path when operated in the discharging mode. Insome embodiments, the dimensions of a single cell may be varied alongits length, such that a cell inlet has a smaller volume than a celloutlet (for example). In alternative embodiments, cells at a first endof a flow path may have uniform, but smaller volume than cells at asecond end of a flow path.

The mass transport rate of a cell can also be varied by selecting porouselectrodes with materials, shapes or other properties that increase ordecrease turbulence within the cells. For example, porous electrodematerials with increased tortuosity will tend to exhibit increasedelectrolyte turbulence and increased mass transport. Overall electrodesurface area may also be varied, such as by varying a diameter ofgraphite fibers making up a porous electrode felt. Increasing electrodesurface area will tend to increase mass transport rates.

Thus, in some embodiments where it is desirable to configure a cascadeflow battery stack assembly to improve its charge reaction, the cellsmay be configured so that the reactant mass transport rate decreases ineach cell along the flow path from inlet to outlet in the dischargemode, but increases in each cell along the flow path from the inlet tothe outlet while in the charging mode. Alternatively, in embodiments forwhich it is desirable to configure a cascade flow battery stack assemblyfor a discharge reaction, the cells may be configured such that thereactant mass transport rate increases in each cell along the flow pathfrom inlet to outlet in the discharge mode, and decreases in each cellalong the flow path from the inlet to the outlet while in the chargingmode.

Electrode residence time is related to the volumetric flow rate ofelectrolytes through a flow battery cell. An increased volumetric flowrate will result in a decreased electrode residence time, and viceversa. In some embodiments, it may be desirable to express electroderesidence time as an optimization parameter for configuring cells. Ahigher electrode residence time is generally preferred for higherexpected reactant concentrations (i.e., higher SOC), in order to allowmore time for electrolytes to react within the cell. A lower electroderesidence time is preferred for lower reactant concentrations in orderto reduce the possibility for parasitic reactions.

Catalysts may be plated, bonded, adhered or otherwise attached to asurface of an electrode in order to increase a rate at which a desiredelectrochemical reaction occurs. Similarly, reaction suppressants may beapplied to an electrode to selectively suppress or reduce the rate atwhich certain undesirable reactions occur within a cell. The degree ofloading and activity of such catalysts or reaction suppressants may beconfigured based on an expected state-of-charge condition of reactantsin a cell and/or the location of the cell within a cascade stage.

In some embodiments, the discharge catalyst loading and dischargecatalyst activity (both Group B design parameters) may be configuredbased on a cell's position in a cascade expected state-of-chargecondition of reactants in the cell. Catalyst loading is defined as thequantity of a catalyst that is plated, bonded, coated or otherwiseprovided on an electrode surface.

Catalyst activity is a measure of the degree of catalysis, or chemicalreaction encouragement of a particular catalyst. Catalyst activity canbe expressed as the ratio of the rate at which a chemical reactionoccurs with the catalyst to the rate at which the same reaction occurswithout the catalyst. Similarly, reaction suppressant activity refers tothe degree to which a material discourages a particular chemicalreaction, and can be expressed in the same terms as catalyst activity.For a given catalyst or suppressant material and a given chemicalreaction, catalyst or suppressant activity can be modified by varyingfactors affecting the rate at which the catalyst or suppressant materialcome into contact with active species within an electrolyte. Suchfactors may include surface treatments, crystal size or structure,particle size, etc.

Thus, as used herein discharge catalyst (or suppressant) activity refersto the reaction modifying strength of catalysts (or suppressants) whichencourage a discharge reaction (or discourage side-reactions duringdischarging) within electrochemical cells of a redox flow battery.Similarly, discharge catalyst loading refers to the quantity of acatalyst (or suppressant) covering a surface of the electrode, where thecatalyst (or suppressant) encourages a discharge reaction (ordiscourages side-reactions during discharging) within electrochemicalcells of a redox flow battery.

In some embodiments, the discharge catalyst (or suppressant) loading anddischarge catalyst (or suppressant) activity may be increased in eachcell along the flow path of redox flow battery stack assembly from inletto outlet in the discharge mode and decreased in each cell along theflow path of redox flow battery stack assembly from inlet to outlet inthe charge mode to compensate for decreasing reactant concentrations, asindicated by the design trend line 1116.

Similarly, in some embodiments the charge catalyst (or suppressant)loading and charge catalyst (or suppressant) activity (both Group Adesign parameters) may be configured based on a cell's position in acascade. Thus, as used herein charge catalyst (or suppressant) activityrefers to the reaction modifying strength of catalysts (or suppressants)which encourage a charging reaction (or discourage side-reactions duringcharging) within electrochemical cells of a redox flow battery.Similarly, charge catalyst loading refers to the quantity of a catalyst(or suppressant) covering a surface of the electrode, where the catalyst(or suppressant) encourages a charge reaction (or discouragesside-reactions during charging) within electrochemical cells of a redoxflow battery.

Thus, in some embodiments, the charge catalyst (or suppressant) loadingand charge catalyst (or suppressant) activity may be decreased in eachcell along the flow path of a redox flow battery stack assembly frominlet to outlet in the discharge mode and increased in each cell alongthe flow path of redox flow battery stack assembly from inlet to outletin the charge mode to compensate for decreasing reactant concentrations,as indicated by the design trend line 1112. The specific catalyst (orsuppressant) loading and activity implemented within each stage in thecascade can be determined using the number of stages in the cascade, thedesign trend line 1116 with respect to discharging, and trend line 1112with respect to charging.

Electrical efficiency of the redox flow battery system may be enhancedby heating the reactants to an elevated temperature, such as about 40°to 65° C., using on-site waste heat from equipment or facility coolingsystems, geothermal heating systems, solar hot water systems, electricalresistance heaters or any other suitable heat source. In someembodiments, a heating fluid may be heated by such a heat source andpumped through a heat exchanger to heat the electrolytes. Alternatively,heaters or heat exchangers may be incorporated directly into stackassemblies, tank enclosures and/or inter-stage components.

In some embodiments, the temperature of electrolytes may be controlledas the electrolytes flow through the cascade redox flow battery stackassembly. FIG. 26 illustrates in design curves 1112 and 1116 how thetemperature may be controlled along the flow path through the cascaderedox flow battery stack assembly. In some embodiments, the temperatureof electrolytes may be controlled by heating or cooling electrolytes instorage tanks, while in other embodiments, the electrolyte temperaturesmay be more discretely controlled by heating or cooling electrolytes ata point just before or just after a cascade stage along a flow path.

The design curve to employ for a given redox flow battery stack assemblymay be based on whether a greater improvement in battery efficiency isachieved by configuring the design to improve performance of thedischarge reactions or the charge reactions. For example, in an Fe/Crsystem, the anolyte charge reaction has the most limited reaction rates,so design trend line 1112 would be selected for the temperature profiledesign parameter. Thus, for a cascade flow battery stack assemblyconfigured for a charge reaction according to trend line 1112,electrolytes will be heated to a higher temperature for reaction instages closer to the inlet, and allowed to cool (or actively cooled) forreactions in stages closer to the outlet during a discharge mode. Forthe same system in a charging mode, electrolytes may be progressivelyheated along the cascade such that reactants are at a lower temperaturein stages closest to an inlet and electrolyte temperatures are higher instages closer to the outlet.

In a similar manner, embodiment redox flow battery cells may beconfigured with different membrane separator materials along the cascadeflow path. Design curve 1112 shown in FIG. 26 illustrates how membraneseparator reactant selectivity may be varied along the cascade flowpath.

Membrane selectivity is defined as the inverse of the reactant transportrate through the separator, and in some embodiments can be expressed inunits of μg Fe/hr-cm/M or cm²/second. In other words, a separator has a“higher” selectivity when it allows a smaller quantity of reactant ionsto pass through the membrane. A separator membrane's selectivitytypically correlates positively with the membrane's area specificresistance (e.g., measured in ohm-cm²), and correlates negatively(inversely) with diffusion rate and ionic conductivity.

A number of physical characteristics of a separator membrane materialare also strongly correlated with selectivity. For example, both asmaller average pore size and a higher pore tortuosity correlate withhigher separator selectivities.

In the discharge mode, cells near the inlet to the cascade redox flowbattery stack assembly will experience electrolyte with a highstate-of-charge and a high concentration of reactants (e.g., Cr 2+ andFe 3+). Consequently, transport of the reactants through the membraneseparator will result in mixing of reactants with concomitant greaterlosses of stored energy than is the case in stages near the cascadeoutlet where the reactant concentrations are diminished. Therefore, flowbatteries with engineered cascade stack assemblies achieve greaterelectrical charge/discharge efficiency by selecting separator membranesthat limit the migration of reactants through the membrane separator toa greater degree in stages at which higher reactant concentrations areexpected (e.g., near the battery inlet during a discharging operation).On the other hand, membrane separator materials that have high membraneselectivity typically also exhibit high ohmic losses (i.e., electricalresistance, which is the inverse of ionic conductivity through themembrane) that increases energy losses through the battery. Thus, it isgenerally desirable to use a separator membrane that has a selectivitywhich is high enough, but no higher than needed for a particular set ofreactant concentrations. These countervailing properties result in thedesign curve 1112 shown in FIG. 26 used to select separator materialsbased upon the location of stages in the reactant flow path.

The selectivity and electrical resistance of separator membranes may bepartially affected and/or described by other physical properties ofseparator membrane materials, such as membrane average pore size andmembrane tortuosity. A membrane's average pore size is a measure of theaverage size (e.g., in microns) of passageways through the membrane. Theaverage pore size of a membrane tends to be inversely correlated withthe membrane's selectivity and electrical resistance. Membranetortuosity is a measure of the complexity of flow paths through themembrane. A high degree of tortuosity represents a complex flow pathwith many twists and turns, and tends to be correlated with a higherdegree of membrane selectivity.

Another variable affecting membrane selectivity is the membrane'sthickness. A thicker membrane layer will also have a higher selectivity,and will result in lower cross-mixing of reactants. In some embodiments,two layers of a separator material may be stacked together toeffectively double the selectivity of a single layer. Similarly, asingle-layer separator material may be selected based in part on itsthickness in order to achieve a desired selectivity.

Thus, in one embodiment, a redox flow battery stack assembly may includecells at one end of the flow path having membrane separators made from amaterial with high membrane selectivity at the cost of greater ohmiclosses, while cells at the other end of the flow path may have membraneseparators made from a material with lower selectivities and therebylower ohmic losses. This design approach may be beneficial partlybecause the driving force for cross mixing is greatly diminished due tothe low concentrations of spontaneously-reacting active species at theoutlet end in the discharge mode and at the inlet end in the chargemode. In the case of an Fe/Cr redox flow battery (FIG. 4) theconcentration of Cr 2+ and Fe 3+ species are at a minimum at the outletend in the discharge mode and at the inlet in the charge mode.

In configuring cells based on position or expected reactant state byvarying parameters discussed above, the cascade redox flow battery stackassembly may be configured so that reactants flow in one direction forcharging and in the other direction for discharging. Additionally, anyof the above configuration approaches may be applied to flow batterysystem designs including two or more separate redox flow battery stackassemblies where at least one stack assembly is configured for chargingand at least one other stack assembly is configured for discharging.

A multi-stage cascade redox flow battery may be configured for charging,discharging, or both. In some embodiments, flow battery stack assembliesmay be arranged in a bi-directional flow arrangement. In abi-directional assembly, the electrolytes are directed through the redoxflow battery stack assembly in one direction in the charging mode and inthe opposite direction in the discharging mode. In such embodiments, oneend of the flow battery flow path (or cascade series) will always be thelow state-of-charge end, and the opposite end will always be the highstate-of-charge end.

In other embodiments, a first stack assembly may be provided forcharging and a second, separate stack assembly may be provided fordischarging. In such embodiments, electrolytes only flow in onedirection through a stack assembly.

In further embodiments, electrolyte reactants may be directed through asingle stack assembly in a single direction for both charging anddischarging. A single-direction flow battery system allows forrelatively fast (e.g., millisecond range) switching between charging anddischarging modes simply by electrically disconnecting the redox flowbattery from the charging power source and connecting it to the load, orvice versa, such as with an electrical switch. In some embodiments, thecharging power source may include the electrical grid.

In such single-direction embodiments (in either single-stack ormulti-stack systems), the state-of-charge gradient across the cascadestages will reverse direction when the battery is switched from acharging to a discharging mode. During a charging operation,electrolytes entering an inlet to a single-direction stack assembly willbe at a relatively low state-of-charge condition, such that theelectrolytes can be converted to a higher state-of-charge condition asthey pass through the flow battery stack assembly and charged. When theelectrolytes exit the stack assembly during a charge operation, theywill be at a higher state-of-charge condition. During a dischargeoperation, the opposite is true: electrolytes will be at a relativelyhigh state-of-charge condition at the inlet to the single-directionstack assembly and at a relatively low state-of-charge condition at theoutlet.

In the case of both single-directional and bi-directional flow stackassemblies, the cells of the cascade stages may be configured as acompromise between charging and discharging. For example, the stage orstages toward the center of the cascade series may be configured for arelatively higher state-of-charge condition than the stages on the outerends of the cascade series as shown for example in FIG. 27A.Alternatively, as shown in FIG. 27B, the stage or stages toward thecenter of the cascade series may be configured for a relatively lowerstate-of-charge condition than the sages on the outer ends of thecascade series.

In some embodiments, as illustrated for example in FIGS. 27C and 27D, aflow battery stack assembly may be configured primarily for either acharging or a discharging operation. FIG. 27C illustrates therelationship between cascade-stage position and optimum state-of-chargefor a stack assembly configured primarily for a discharge operation. Insuch an embodiment, the cells in stages closer to the cascade seriesinlet will be configured for a higher state-of-charge condition thancells in stages located at an outlet of the cascade series. A stackassembly of this type may be configured for single-directional orbi-directional flow of electrolytes.

Similarly, FIG. 27D illustrates the relationship between cascade-stageposition and optimum state-of-charge for a stack assembly configuredprimarily for a charging operation. In this embodiment, cells in stagescloser to the cascade series inlet may be configured for a relativelylower state-of-charge condition than cells at the outlet of the cascadeseries. A stack assembly of this type may be configured forsingle-directional or bi-directional flow of electrolytes.

Hydraulically, electrolyte flows in series from one cascade stage to thenext. Between stages, flow batteries may be constructed to include anearly infinite variety of flow paths, sensors, devices, etc. Forexample, in some embodiments, electrolyte exiting a first cell in afirst stage can flow directly into a first cell of a second stagewithout mixing with electrolyte from any other source. Such embodimentsare referred to herein as “direct cascade” systems. In otherembodiments, electrolyte exiting a first cell of a first stage can bemixed with electrolyte exiting a second cell of the first stage orelectrolyte from other sources before being directed into one or morecells of a second stage.

While the above description refers to a cascade flow battery stackassembly in a linear orientation, the skilled artisan will recognizethat such a system may be arranged in any physical orientation whilemaintaining a plurality of stages in hydraulic series.

FIG. 28 illustrates an embodiment of a direct cascade flow battery cellblock 1340. The illustrated embodiment includes three independentsingle-cell stages 1342, 1344, 1346 arranged in a single layer 1348. Inthis embodiment, electrolyte flows directly from the first cell 1342 tothe second cell 1344 and to the third cell 1346 while remaining in thelayer 1348, without mixing with electrolyte in other layers betweencascade stages. Additional independent layers 1348 may be added andelectrically connected to one another as needed to obtain desired poweroutput or energy storage needs. In some embodiments, hydraulic flowchannels between adjacent stages (e.g., 1342 & 1344 or 1344 & 1346) maybe simple linear direct flow channels. In alternative embodiments, flowchannels between adjacent stages may include complex, tortuous paths,measuring or monitoring equipment or any number of other elements. Adirect cascade flow battery system such as that shown in FIG. 28 mayinclude any number of stages in a single layer.

FIG. 29 schematically illustrates an embodiment of a redox flow batterystack assembly 1350 made up of a plurality of direct cascade sections.In some embodiments, each direct cascade section may include a singlecell in each stage. In some embodiments, the single-cell stages may bearranged in layers, such as illustrated in FIG. 28 or FIG. 6. In otherembodiments, the direct cascade sections may be stacked vertically orotherwise.

In the embodiment illustrated in FIG. 29, electrolyte enters the stackassembly 1350 at an inlet 1352. Electrolyte is then directed through twodirect cascade sections 1354 a, 1354 b in parallel. After exiting thefirst two direct cascade sections, electrolyte may be directed to anout-flow channel 1356. From the inflow channel 1352, electrolyte alsoflows through a first shunt breaker 1358 a and then into a second pairof direct cascade sections 1354 c, 1354 d. After exiting the second pairof direct cascade sections, electrolyte may be directed through a secondshunt breaker 1358 b. In the same way, electrolyte also flows from theinlet channel 1352 through a third shunt breaker 1358 c, through a thirdpair of direct cascade sections 1354 e, 1354 f, through a fourth shuntbreaker 1358 d, and through the exit channel 1356.

In the illustrated embodiment, shunt breakers 1358 advantageouslyisolate pairs of direct cascades (which may include more than threestages as needed), thereby limiting shunt currents through the stackassembly 1350. For example, in the illustrated embodiment, electrolytemay be simultaneously directed through two three-stage cascades beforebeing directed to the outflow channel. Placing shunt breakers in boththe in-flow and out-flow channels will reduce shunt currents. Inalternative embodiments, each single direct cascade section may beisolated by shunt breakers. In further alternative embodiments, directcascades may be combined into groups of three or more and isolated fromother sections of the stack assembly by shunt breakers.

In some embodiments, common inlet and outlet flow channels may be sizedto minimize shunt currents while allowing consistent flow rates throughthe direct cascade sections. The flow channels directing electrolytethrough the cells of the direct cascade sections may also be configuredto limit shunt currents within a direct-cascade section or blocks ofdirect cascades.

In some embodiments, each stage of the cascade stack may be configuredaccording to its position along the flow path by optimizing cellcharacteristics for the state-of-charge of the electrolyte(s) at eachstage, as discussed herein.

In some embodiments, the shunt breakers may be configured tosubstantially reduce or eliminate shunt currents by creating a highelectrical resistance in the electrolyte flow path. Various embodimentsof suitable shunt breakers are discussed below, but generally anystructure capable of substantially reducing or eliminating shuntcurrents in an electrolyte fluid may be used as a shunt breaker. Shuntcurrents are a form of short-circuit caused by electrical currentsflowing through the liquid electrolyte resulting in a consumption ofavailable discharge energy or delivered charge energy. It is desirableto limit shunt currents to minimize the negative impacts on systemefficiency.

FIG. 30 schematically illustrates an embodiment of an alternate flowpath arrangement for a multi-stage cascade flow battery. The flow patharrangement illustrated in FIG. 30 is referred to as a “Z-flow”arrangement. The Z-flow arrangement includes multiple cascaded stages1052, 1054, 1056 with a plurality of cells in each stage. In theembodiment of FIG. 30, the adjacent stages 1052, 1054, 1056 may beseparated by inter-stage components 1152 a, 1152 b which may includeseveral functional elements such as “shunt breakers” designed tosubstantially reduce or entirely prevent electrical shunt currents fromflowing between stages.

In the Z-flow arrangement illustrated schematically in FIG. 30,electrolyte is directed through a single stage 1052 such that flowthrough the cells 1049 of a single stage 1052 occurs in parallel. Forexample, in the illustrated embodiment, a first stage 1052 includes aplurality of cells 1049, each having an inlet 1154 on the right side andan outlet 1156 on the left side such that electrolyte flows through eachof the first-stage cells in the same direction from right to left. Uponexiting the first stage 1052, electrolyte may be directed into the firstinter-stage component 1152 a. After leaving the inter-stage component1152 a, the electrolyte may be directed into the second stage 1054 whichis illustrated with inlets 1154 on the left side of each cell andoutlets 1156 on the right side. Once the electrolyte exits the secondstage 1054, it may be directed into the second inter-stage component1152 b and then into the third stage 1056 of the cascade. In someembodiments, electrolyte may be directed into a storage tank afterexiting the final stage of the cascade.

The name “Z-Flow” is derived from the illustrated schematic and ismerely used for convenience to differentiate the arrangement. The nameis not intended to limit the scope of the arrangement to any particularphysical shape or configuration of elements. The skilled artisan willrecognize that the actual spatial orientation of the cells in a Z-flowstack assembly may be arranged in any number of other ways as needed fora particular enclosure, application or other design requirements. Forexample, in some embodiments, the cells will be positioned in a verticalorientation with inlets at a bottom end and outlets at a top end. Such avertical orientation has the additional advantage of allowing gasbubbles to be released near the outlet of each cell or each stage. Aswill also be clear to those skilled in the art, it is not necessary foradjacent stages in a Z-flow arrangement to have alternating inlets andoutlets. For example, the arrangement illustrated in FIG. 30 could bemodified such that electrolyte flows from left-to-right through all ofthe three stages by providing an additional flow channel segment inbetween stages to re-direct flow from the left side to the right side ofthe cells.

The flow battery stack assembly illustrated in FIG. 30 includes threestages with ten cells in each stage. However, any number of stages andcells per stage may be used. For example, a cascade flow battery stackassembly may be arranged with two, three, four, five, six, seven or morestages, each having any number of cells as needed to achieve a desiredpower output or input. In many embodiments, cascade flow batteries willhave the same number of cells in each stage, but this is not arequirement as a cascade flow battery stack assembly may be designedwith different numbers of cells in each stage.

In a direct cascade system, such as that shown in FIG. 28, (in which thestages are joined directly without shunt breakers or other inter-stagecomponents), shunt losses increase in proportion to an increasing numberof cells in each stage. As a practical matter, the number of cells ineach stage may often be high in order to achieve high input and/oroutput voltages. A Z-flow arrangement may substantially improve theefficiency of a cascade redox flow battery system by substantiallyisolating each stage (or cell block) from shunt currents to/from otherstages. The Z-flow arrangement allows easier management of a flowbattery system including a substantial number of stages joined inhydraulic series.

In some embodiments, inter-stage components 1152 include one or morefunctional components such as shunt breakers, re-directing flow channels(e.g., to direct flow from the top of one cell block to the bottom ofthe next), state-of-charge sensors, electrical current sensors,electrical resistance sensors, volt meters, density sensors,spectroscopic sensors, OCV sensors, reactant balance sensors, de-gassingvalves, heat exchangers, heaters, sight glasses, cooling devices, flowmeters, pressure sensors, pressure relief valves, pressure mediators,pumping devices, etc.

Shunt breakers may include any components capable of substantiallyreducing or eliminating shunt currents in a fluid electrolyte. In someembodiments, a shunt breaker includes a long flow channel withsufficient length to create a large electrical resistance between stacksconnected by the flow channel. For example, in some embodiments, shuntbreakers with flow channels of more than 35 cm have been found to besuitable. Shunt breakers with flow channels of 150 cm have been found toreduce shunt currents by as much as 50% relative to arrangements with noshunt breakers in the same 3-stage, 10-cell Z-flow arrangement. In otherembodiments, a shunt breaker may include a very narrow or small-diameterflow channel in addition to or in combination with a long flow channelto achieve the same objective.

Other shunt current structures suitable for mitigating or breaking shuntcurrents may also be used, such as structures which create a physicaland/or electrical discontinuity or otherwise create a substantially highelectrical resistance between an upstream and a downstream section of aflow path. One example of a structure for creating a physicaldiscontinuity in an electrolyte flow path is described in U.S. Pat. No.6,475,661, which describes a drip column for creating a discontinuity ina flowing electrolyte.

In some embodiments, a peristaltic pump, which advances fluid bycompressing a section of flexible tubing, can be used as both a shuntbreaker and as a metering pump between stages of a cascade flow batterysystem. In such embodiments, the electrolyte is isolated in pocketsformed between pinched-together sections of the tubing, with the tubingmaterial (typically plastic or rubber compound) providing a highresistance path between isolated pockets of electrolyte. Two examples ofperistaltic pumps are illustrated and described in U.S. Pat. Nos.4,522,571 and 4,702,679, the entire contents of both of which are herebyincorporated by reference. Many other peristaltic pumps are also knownto those skilled in the art.

A variety of reactants and catalysts may be used in the redox flowbattery system. One embodiment set of electrolyte reactants is basedupon the iron and chromium reactions that is illustrated in FIG. 4. Thereactants in the Fe/Cr redox flow battery system stores energy in FeCl 3(Fe 3+) in the catholyte, which reacts at the positive electrode, andCrCl 2 (Cr 2+) in the anolyte, which reacts at the negative electrodewithin cells of the battery.

An undesirable non-faradic electron transfer reaction can occur betweenFe 3+ and Cr 2+ if these ions come into proximity to one another. Thenon-faradic electron transfer reaction reduces the system's coulombicefficiency within a given charge-discharge cycle. Therefore, to maintaina high level of coulombic efficiency, electrolyte cross-mixing within acharged Fe/Cr redox flow battery stack assembly should be minimized. Oneway to minimize electrolyte cross-mixing is to use a highly selectivemembrane separator 12, such as Nafion®-117 ion-exchange membrane(DuPont, USA), that eliminates or severely limits transport of Fe 3+ andCr 2+ through its cross-section. A disadvantage of highly-selectivemembrane separators is that they have low ionic conductivity whichresults in lower voltage efficiency within the redox flow battery stackassembly. Additionally, ion-exchange membranes are expensive, with aprice in the neighborhood of $500/m². Additionally, long-term stabilityand increased crossover of “other ions” (e.g., Fe 3+ and Cr 2+) canoccur at elevated operating temperatures (e.g., 35° C. and higher).Since the DC energy storage efficiency of a redox flow battery is theproduct of coulombic and voltage efficiencies, an optimization tradeoffexists.

An embodiment of a flow battery system using the Fe/Cr system is knownas a mixed reactant system where FeCl 2 (Fe 2+) is added to the anolyteand CrCl 3 (Cr 3+) is added to the catholyte, as described in U.S. Pat.No. 4,543,302, the entire contents of which are incorporated herein byreference. An advantage of the mixed reactant system is that thedischarged anolyte and discharged catholyte are identical. Further, whenthe total concentration of Fe in the anolyte is the same as thecatholyte, and the total concentration of Cr in the catholyte is thesame as the anolyte, the concentration gradients across the membraneseparators 12 may be eliminated. In this way the driving force forcross-mixing between anolyte and catholyte may be reduced. When thedriving force for cross-mixing is reduced, less selective membraneseparators may be used, thereby providing lower ionic resistance andlower system costs. Examples of less-selective membrane separatorsinclude microporous membrane separators manufactured by Celgard LLC, andmembrane separators made by Daramic LLC, both of which cost in theneighborhood of $5/m² to $10/m². By optimizing the cell characteristicsfor the reactant state of charge at each cascade stage and completingthe charge or discharge in one pass, the embodiments described hereinprovide suitably high efficiency in a redox flow battery stack assemblyincluding materials that are approximately two orders of magnitude lowerin cost than in conventional redox flow battery designs.

The benefits of the iron/chromium mixed reactant embodiment discussedabove can also be enjoyed by applying the engineered cascade systemembodiments to any other flow battery chemistry in which the chemicalcompositions of both electrolytes at one end of the state of chargerange (e.g., discharged or charged) is exactly the same. For example,with other known flow battery chemistries it is possible to mixelectrolytes in such a way that the discharged electrolytes arechemically identical, thus substantially reducing inefficiencies causedby migration of small amounts of electrolyte across the separatormembrane, and allowing for the use of less-selective, lower costseparator membrane materials.

Other known flow battery chemistries include V(II)/V(III) (anolyte) andV(IV)/V(V) (catholyte), Sn (anolyte)/Fe (catholyte), Mn (anolyte)/Fe(catholyte), Ti (anolyte)/Fe (catholyte), V (anolyte)/Ce (catholyte), V(anolyte)/Br 2 (catholyte), Fe (anolyte)/Br 2 (catholyte), Cr(anolyte)/Br 2 (catholyte), and S (anolyte)/Br 2 (catholyte). An exampleof an alternate common discharged electrolyte system is a chargedanolyte including Sn 2+ and Fe 2+ and charged catholyte including Fe 3+and Sn 4+, in which the composition of both discharged electrolytes isSn 4+ and Fe 2+. A second example of an alternate common dischargedelectrolyte system is a charged anolyte including Ti 2+ and Fe 2+ andcharged catholyte including Fe 3+ and Ti 3+, in which the composition ofboth discharged electrolytes is Ti 3+ and Fe 2+. Another example of acommon discharged electrolyte system includes a charged anolyteincluding V 3+ and Fe 2+, in which the composition of both dischargedelectrolytes is V 3+ and Fe 2+. Other common discharged electrolytesystems are also possible.

Another example of an alternate discharged electrolyte is a redox flowbattery with semi-solid reactants as described in Chiang et al., U.S.Pat. No. 7,338,734, where the active material in the anolyte andcatholyte in the discharged state is based on LiMn₂O₄ (lithium manganesespinel). In this system upon charging, Li ions will be extracted fromthe LiMn₂O₄-containing particles in contact with the positive electrode,raising the potential, and inserted into the LiMn₂O₄-containingparticles in contact with the negative electrode, lowering thepotential. The potential difference between the anolyte and catholytewill rise from 0 to approximately 1.5V. Variants of LiMn₂O₄ in which anelement partially substitutes for the Mn may also be used.

The various system embodiments may employ a variety of electrolytestorage tank configurations, such as those described above withreference to FIG. 1, 14, 15 or 17-20, as well as other tankconfigurations. For example, in a simple embodiment, a single tank maybe used to store each electrolyte as illustrated in FIG. 1. Thistwo-tank configuration reduces the number of tanks and may enable rapidswitching from charge to discharge modes (and vice versa). However, suchembodiments can suffer from efficiency losses from mixing of charged anddischarged electrolytes. Alternatively, 3-tank and 4-tank arrangementsmay be employed. Further alternative embodiments include tanks withseparators which effectively divide a single tank into two electrolytestorage compartments are described above with reference to FIGS. 19A-19Fand 20A-20F.

The voltage of a flow battery, and its concomitant power rating, may beadjusted by varying the number of cells in each stage. However, largenumbers of cells in a stage can result in shunt currents through theconductive anolyte and catholyte that are parasitic to the flowbattery's energy storage efficiency. Alternatively, the power rating ofa flow battery may be raised without altering the voltage rating byincreasing the area (e.g., m²) or the current density (e.g., mA/cm²) ofthe cells within one or more cascade stages, thereby increasing thetotal battery current. Thus, the number of stages, the number of cellsin each stage, and the design configurations applied to each cell may bevaried based upon the design goals and performance requirements of aparticular flow battery system.

One beneficial characteristic of a flow battery including a cascadestack assembly is that the operating voltage is relatively flat due tothe constant reactant profiles within each cascade stage during chargeand discharge. Steady operating voltages may be advantageous in someembodiments as this may reduce the number of capacitors required withina flow battery's power conditioning system (PCS) that converts thedirect current (DC) electricity output to alternating current (AC)electricity. Reducing the number of capacitors within a PCS reduces itscost, increases its reliability, and increases its efficiency.

Typically, for the same general operating conditions (e.g., currentdensity, flow rate, temperature, etc.) the charge voltage from a givencollection of cells is higher than the discharge voltage due to internalresistances. To achieve more similar charge and discharge voltages, thecharge voltage can be reduced by lowering the reactant flow rate duringcharge to reduce current density. Alternatively, a larger number ofseries-connected cells within each stage may be used when the flowbattery is in discharge mode. In a bi-direction stack assembly, this maybe achieved by incorporating appropriate valve arrangements or otherflow bypass mechanisms. Operating voltage may also be tuned by providinga plurality of engineered cascade stack assemblies dedicated to thedischarge process in addition to one or more separate engineered cascadestack assemblies dedicated to charging. To make operating voltages moresimilar, the operating current density, the number of cells, or the areaof each cell in the discharge stack assembly may be increased relativeto the charge stack assembly.

Embodiments of flow battery systems utilizing the engineered cascadesystems of the various embodiments provide several advantages overnon-cascade or non-engineered cascade flow battery systems. For example,as shown in FIG. 31, a redox flow battery system with an engineeredcascade stack assembly enjoys a substantially increased efficiency overa broader range of current densities as compared with “legacy” systems(i.e., non-cascade or non-engineered cascades). The Legacy Type A andLegacy Type B lines represent two different flow battery arrangements,each including a 3-stage cascade in which each of the three stagesincludes cells identical to the cells in other stages. As shown in FIG.31, an engineered cascade system may be configured to operate at greaterthan 70% efficiency at current densities of up to 160 mA/cm2.

Embodiments of redox flow battery cells, stack assemblies and systemsdescribed herein may be used with any electrochemical reactantcombinations that include reactants dissolved in an electrolyte. Oneexample is a stack assembly containing the vanadium reactantsV(II)/V(III) or V 2+/V 3+ at the negative electrode (anolyte) andV(IV)/V(V) or V 4+/V 5+ at the positive electrode (catholyte). Theanolyte and catholyte reactants in such a system may be dissolved insulfuric acid. This type of battery is often called the all-vanadiumbattery because both the anolyte and catholyte contain vanadium species.Other combinations of reactants in a flow battery that can utilize thefeatures and advantages of the systems described herein include Sn(anolyte)/Fe (catholyte), Mn (anolyte)/Fe (catholyte), V (anolyte)/Ce(catholyte), V (anolyte)/Br 2 (catholyte), Fe (anolyte)/Br 2(catholyte), Ti (anolyte)/Fe (catholyte), and S (anolyte)/Br 2(catholyte). In each of these example chemistries, the reactants may bepresent as dissolved ionic species in the electrolytes, which permitsthe advantageous use of engineered cascade flow battery cell and stackassembly designs in which cells have different physical, chemical orelectrochemical properties at different expected state of chargeconditions along the cascade flow path (e.g., cell size, type ofmembrane or separator, type and amount of catalyst, etc.). A furtherexample of a workable redox flow battery chemistry and system isprovided in U.S. Pat. No. 6,475,661, which is incorporated herein byreference.

The foregoing description of the various embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, and instead theclaims should be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

We claim:
 1. A redox flow battery energy storage system, comprising: afirst redox flow battery cell block; and a second redox flow batterycell block, the first redox flow battery cell block having an outletfluidically coupled to an inlet of the second redox flow battery cellblock, wherein the first cell block has a first structural configurationbased on a first state-of-charge range and the second cell block has asecond structural configuration different from the first structuralconfiguration, the second structural configuration based on a secondstate-of-charge range that is different from the first state of chargerange.
 2. A redox flow battery energy storage system, comprising: afirst redox flow battery cell block configured to operate at a firststate of charge range, the first redox flow battery cell blockcomprising: a first plurality of cells each of which has at least oneof: a first structural property and a first material property, the atleast one of the first structural property and the first materialproperty based on the first state of charge range; and a second redoxflow battery cell block configured to operate at a second state ofcharge range different than the first state of charge range, andcomprising a second plurality of cells each of which has at least oneof: a second structural property and a second material property, the atleast one of the second structural property and the second materialproperty based on the second state of charge range, the at least one of:the second structural property and the second material property beingdifferent from the at least one of: the first structural property andthe first material property, wherein the first redox flow battery cellblock is fluidically coupled in a cascade arrangement along a reactantflow path to the second redox flow battery cell block.
 3. A redox flowbattery energy storage system, comprising: a plurality of redox flowbattery cell blocks fluidically coupled together in fluidic series toform a reactant flow path, an outlet of a first of the plurality of cellblocks coupled to an inlet of a second one of the plurality of cellblocks along the reactant flow path, the plurality of cell blocksoperating according to the same one of: a charge mode and a dischargemode; wherein: each of the redox flow battery cell blocks comprises aplurality of cells, each of the cell blocks has a physical configurationaccording to a state of charge of reactant in the respective cellblocks; the first one of the plurality cell blocks has a first physicalconfiguration according to a first state of charge range; the second oneof the plurality of cell blocks has a second physical configurationaccording to a second state of charge range; the first state of chargerange and the second state of charge range are different; and the firstphysical configuration of the first one of the plurality of cell blocksand the second physical configuration of the second one of the pluralityof cell blocks are different.
 4. The redox flow battery energy storagesystem of claim 3, wherein the plurality of redox flow battery cellblocks are fluidically coupled together so that reactant flows througheach of the plurality of redox flow battery cell blocks one at a time.5. The redox flow battery energy storage system of claim 4, wherein: theplurality of redox flow battery cell blocks are fluidically coupledtogether so that reactant separates to flow in parallel through theplurality of cells in each redox flow battery cell block and remixesbetween each of the plurality of redox flow battery cell blocks.
 6. Theredox flow battery energy storage system of claim 5, wherein each of theplurality of redox flow battery cell blocks further comprises aplurality of shunt breakers arranged between at least some of theplurality of cells in each redox flow battery cell block.
 7. The redoxflow battery energy storage system of claim 5, further comprising aplurality of inter-stage components fluidically coupled between each ofthe plurality of redox flow battery cell blocks.
 8. The redox flowbattery energy storage system of claim 7, wherein the plurality ofinter-stage components comprises a plurality of shunt breakersconfigured to reduce shunt currents flowing in the plurality of redoxflow battery cell blocks.
 9. The redox flow battery energy storagesystem of claim 8, wherein the plurality of shunt breakers comprise aperistaltic pump.
 10. The redox flow battery energy storage system ofclaim 7, wherein the plurality of inter-stage components comprises aplurality of sensors.
 11. The redox flow battery energy storage systemof claim 10, wherein the plurality of sensors includes sensors selectedfrom the group comprising state-of-charge sensors, electrical currentsensors, electrical resistance sensors, volt meters, density sensors,spectroscopic sensors, OCV sensors, reactant balance sensors, flowmeters, and pressure sensors.
 12. The redox flow battery energy storagesystem of claim 7, wherein the plurality of inter-stage componentscomprises a plurality of valves.
 13. An electrical power system,comprising: a source of electrical power; and a redox flow batterysystem configured to receive electrical power from the source ofelectrical power and provide electrical power to an electrical load, theredox flow battery system comprising: a plurality of redox flow batterycell blocks fluidically coupled together in fluidic series to form areactant flow path, an outlet of a first of the plurality of cell blockscoupled to an inlet of a second one of the plurality of cell blocksalong the reactant flow path, the plurality of cell blocks operatingaccording to the same one of: a charge mode and a discharge mode;wherein: each of the redox flow battery cell blocks comprises aplurality of cells, each of the cell blocks has a physical configurationaccording to a state of charge of reactant in the respective cellsblocks; the first one of the plurality cell blocks has a first physicalconfiguration according to a first state of charge range; the second oneof the plurality of cell blocks has a second physical configurationaccording to a second state of charge range; the first state of chargerange and the second state of charge range are different; and the firstphysical configuration of the first one of the plurality of cell blocksand the second physical configuration of the second one of the pluralityof cell blocks are different.
 14. The electrical power system of claim13, wherein the plurality of redox flow battery cell blocks arefluidically coupled together so that reactant flows through each of theplurality of redox flow battery cell blocks one at a time.
 15. Theelectrical power system of claim 14, wherein: the plurality of redoxflow battery cell blocks are fluidically coupled together so thatreactant separates to flow in parallel through the plurality of cells ineach redox flow battery cell block and remixes between each of theplurality of redox flow battery cell blocks.
 16. The electrical powersystem of claim 15, wherein each of the plurality of redox flow batterycell blocks further comprises a plurality of shunt breakers arrangedbetween at least some of the plurality of cells in each redox flowbattery cell block.
 17. The electrical power system of claim 15, furthercomprising a plurality of inter-stage components fluidically coupledbetween each of the plurality of redox flow battery cell blocks.
 18. Theelectrical power system of claim 17, wherein the plurality ofinter-stage components comprises a plurality of shunt breakersconfigured to reduce shunt currents flowing in the plurality of redoxflow battery cell blocks.
 19. The electrical power storage system ofclaim 18, wherein the plurality of shunt breakers comprise a peristalticpump.
 20. The electrical power system of claim 17, wherein the pluralityof inter-stage components comprises a plurality of sensors.
 21. Theelectrical power system of claim 20, wherein the plurality of sensorsincludes sensors selected from the group comprising state-of-chargesensors, electrical current sensors, electrical resistance sensors, voltmeters, density sensors, spectroscopic sensors, OCV sensors, reactantbalance sensors, flow meters, and pressure sensors.
 22. The electricalpower system of claim 17, wherein the plurality of inter-stagecomponents comprises a plurality of valves.